Traxxas TRX 2.5R Owner's Manual

OWNERS MANUAL

™

®

TRX 2.5R •2

Traxxas
1100 Klein Road
Plano, Texas 75074
Phone: [phone]
Toll-free 1-888-TRAXXAS

Traxxas U.K.
P.O. Box 1128
Winterbourne, Bristol BS36-2SH, England
Phone: [phone]

Internet
www.Traxxas.com
e-mail: [e-mail]

Entire contents ©2004 Traxxas. Traxxas, TRX 2.5R, Top Fuel, and Ready-To-Race are trademarks
or registered trademarks of Traxxas. Other brand names and marks are the property of their
respective holders and are used only for purposes of identification. No part of this manual may
be reproduced or distributed in print or electronic media without the express written
permission of Traxxas.

Note: Some photos show the engine equipped with optional accessories, such as the EZ-Start

system, headers, and engine mounts, that are not included in the engine package. See the end
panel of the engine box for exact contents based on your engine model number.

TRX 2.5R •3

Before You Proceed

Carefully read and follow all instructions in this and any accompanying materials to prevent
serious damage to your engine. Failure to follow these instructions will be considered abuse
and/or neglect. Before running your TRX 2.5R Racing Engine, look over this entire manual and
examine it carefully. If for some reason you decide the TRX 2.5R is not what you wanted, do
not continue any further.

Your hobby dealer absolutely cannot accept a TRX 2.5R for return

or exchange after it has been run.
Warnings, hints, and cross-references

Throughout this manual, you’ll notice warnings and helpful hints, identified by the icons
below. Be sure to read them!

An important warning about personal safety or avoiding damage
to your TRX 2.5R Racing Engine and related components.

Special advice from Traxxas to make things easier and more fun.

Refers you to a page with a related topic.

Support

If you have any questions about your TRX 2.5R Racing Engine or its operation, call the Traxxas
Technical Support line toll-free at: 1-888-TRAXXAS (1-888-872-9927)

Technical support is available Monday through Friday from 8:30am to 9:00pm Central time.
Technical assistance is also available at www.Traxxas.com. You may also e-mail customer
support with your question at [e-mail]. Join thousands of Traxxas R/C
enthusiasts in our online community at www.Traxxas.com.

Traxxas offers a full-service, on-site repair facility to handle any of your Traxxas service needs.
Maintenance and replacement parts may be purchased directly from Traxxas, or you can save
shipping and handling costs by purchasing them from your local hobby dealer.

4 Safety Precautions

5 Introduction

6 TRX 2.5R Racing

Engine Illustrated

7 Terms to Know

11 The Fuel

13 The Air Filter

14 The Carburetor

18 Breaking-in Your

TRX 2.5R Racing
Engine

22 Tuning the TRX

2.5R for Best
Performance

27 Troubleshooting

Your TRX 2.5R
Racing Engine

30 Maintaining and

Storing Your TRX

2.5R Racing Engine

33 Index

34 Rebuilding the TRX

2.5R Racing Engine

BEFORE YOU PROCEED

TRX 2.5R •4

All instructions and
precautions outlined in
this manual should be
strictly followed to ensure
safe operation of your
TRX 2.5R Racing Engine.

The TRX 2.5R Racing Engine
is not intended for use by
those under 16 years of
age without the supervision
of a responsible and
knowledgeable adult.

Safety Precautions

All of us at Traxxas want you to safely enjoy your new TRX 2.5R Racing Engine. Operate your TRX

2.5R sensibly and with care, and it will be exciting, safe, and fun for you and those around you.
Failure to operate your TRX 2.5R in a safe and responsible manner may result in property damage
and serious injury. The precautions outlined in this manual should be strictly followed to help
ensure safe operation. You alone must see that the instructions are followed and the precautions
are adhered to.

Important Points to Remember

• R/C fuel is dangerous and highly poisonous. Always follow all directions and precautions printed
on the fuel container.

• R/C fuel, especially when in a fuel dispensing bottle, may look like a cool drink to a child.

Keep all

fuel out of the reach of children at all times.

Do not place fuel containers on the ground where

children can get to them while you are driving.

• R/C fuel is flammable. Never allow smoking, sparks, heat or flame in the presence of fuel
or fuel vapors.

• The engine and exhaust system may become extremely hot during use. Be careful not to touch
the parts, especially when refueling or stopping the engine.

• Prolonged exposure to the engine exhaust can be harmful. Avoid breathing the engine exhaust.
Always run your TRX 2.5R outdoors, in a well-ventilated area. Never run the engine indoors.

• Don’t operate your model at night or anytime your line of sight to the model may be obstructed
or impaired in any way.

• Never operate your model in crowds of people or busy pedestrian areas. The TRX 2.5R makes
models extremely fast and may cause injury to those unaware of its presence.

• The engine can be loud. If the noise makes you uncomfortable, wear ear protection. Be
considerate of your neighbors by not running your model early in the morning or late
in the evening.

•

Most importantly, use good common sense at all times.

SAFETY PRECAUTIONS

TRX 2.5R •5

Introduction

The TRX 2.5R Racing Engine is vastly superior to
typical R/C engines and belongs in the category of
specialty high-performance racing engines. It’s the
result of years of focused engineering development,
and thousands of hours of controlled testing and
rigorous real-world running. It offers unprecedented
power and performance that turns ready-to-run into
Ready-To-Race

®

.

The TRX 2.5R Racing Engine takes a total-system
approach. Each part of the engine, from the air filter
to the exhaust tip, is carefully engineered to work in
harmony with other engine components. Each part
complements the next, to extract maximum power.
The TRX 2.5R is designed to be tolerant of variations in tuning and running through a
wide range of variable atmospheric conditions, such as changes in temperature, humidity,
and altitude.

The TRX 2.5R is manufactured to exacting tolerances and requires a specially-designed break­in procedure to accomplish the final precision fitting of the internal engine components. It is
very important that you follow the new break-in procedure as closely as possible to achieve
the best performance and longest life from your TRX 2.5R Racing Engine. Old style break-in
procedures, such as simply running the engine with a very rich fuel mixture for the first 4
tanks of fuel, will not achieve the best results. Follow the steps in this manual to successfully
break in the TRX 2.5R Racing Engine.

A routine maintenance schedule is also required to keep the TRX 2.5R in top running
condition. Cleaning the air filter after each hour of running is particularly important. Also it is
extremely important to perform after-run maintenance on the engine to prevent corrosion of
the internal engine components (see page 30).

Traxxas strongly
discourages changing or
modifying any part of the
TRX 2.5R Racing Engine.
Old tech tips and tricks
that may have boosted
the power of other
engines could seriously
diminish the performance
of the TRX 2.5R.

There’s more advanced
thinking, development
and testing in the stock
parts of your TRX 2.5R
than in many aftermarket
manufacturer’s so-called
performance parts. The
TRX 2.5R is already one of
the most powerful
engines in its class and
simply will not benefit
from average, low-tech,
aftermarket bolt on
performance items.

INTRODUCTION

TRX 2.5R •6

TRX 2.5R ILLUSTRATED

Air Filter

Base

Head Protector

PowerTune

™

Head

(Cylinder Head)

Pull Starter

Low-Speed

Needle

Throttle Arm

Clutch Bell

Flywheel

Carburetor

Air Intake

Fuel Intake

High-Speed
Needle

Idle Speed
Screw

Crankshaft

Crankcase

Air Filter
Housing

TRX 2.5R Racing Engine Illustrated

Optional

Backplate

TRX 2.5R •7

TERMS TO KNOW

Terms to Know

You’ll find these Nitro R/C engine terms
throughout this section of the manual.

.15 - .15 or “15” refers to the size of the engine.

The TRX 2.5R is .15 cubic inches or 2.5 cubic
centimeters (cc). The name “TRX 2.5R” is
derived from the cc measurement.

ABC - Abbreviation for aluminum, brass, and

chrome. Refers to engine construction that
consists of an aluminum piston that slides in
a chrome-plated brass sleeve. The TRX 2.5R
uses ABC construction.

Air filter - The air filter sits atop the carburetor

and prevents harmful dust and dirt from
entering the engine. Dirt ingestion is the
number one cause of premature engine
failure so the engine should never be run
without the air filter in place.

Big Block .15 - A .15 (2.5) engine that uses a

larger crankcase size and longer stroke than
typical .15 engines. Generally only used for
specific racing classes.

Break-in - Break-in is the procedure for running

a brand new engine according to specific
instructions. This correctly prepares the
engine for normal running. The break-in
procedure can be different for different
makes of engines. Follow the Traxxas
directions for break-in exactly.

Carb - Abbreviation for carburetor.
Carburetor - The carburetor atomizes (mixes)

the fuel with the air so that the engine can
burn it. There are two types of carburetors;
slide carbs and barrel carbs. The TRX 2.5R
uses the superior slide carburetor design.

Clean-out - Cleaning-out is a condition that

occurs when the engine is accelerating and
the fuel mixture becomes sufficiently lean to
allow the engine to continue into its upper
RPM power band. It is usually characterized by
a noticeable decrease in blue exhaust smoke
and a dramatic increase in engine speed.

Combustion chamber - The combustion

chamber is machined into the bottom of the
cylinder head. This is where the glow plug
ignites the fuel. The shape of the combustion
chamber is designed to promote more
efficient burning of the fuel.

Connecting rod - The connecting rod

transfers the piston motion to the
crankshaft. The TRX 2.5R Racing Engine
uses a “knife-edged” connecting rod.
The aerodynamic, sharpened edges allow
it to “slice” through the pressurized air/fuel
mixture inside the crankcase.

Crankcase - The engine “body” that contains all

of the running mechanical components.

Crankshaft - The main shaft of the engine that

holds the reciprocating assembly.

TRX 2.5R •8

TERMS TO KNOW

Cooling fins - The cooling fins are milled into

the cylinder head and crankcase and cause
heat to be drawn away from the engine. Heat
is removed when it dissipates into the air
passing across the cooling fins. It is important
to keep the fins clean of dirt
and debris for maximum cooling efficiency.

Cylinder head (head) - The finned aluminum

part on top of the engine that is responsible
for dissipating most of the engine’s heat. The
combustion chamber is machined into the
bottom of the head.

Dyno - Abbreviation for dynamometer. A

precise piece of testing equipment that
accurately measures engine power and
torque output over the engine’s entire
RPM range.

EZ-Start - Traxxas on-board electric starting

system. The system consists of a hand held
starter control unit and an on-board gearbox
with an electric motor to spin the engine.

Filter foam - The oiled foam element inside the

air filter housing. The filter foam in the TRX

2.5R must be thoroughly cleaned and re-oiled
after every hour of run time.

Fit - Usually refers to the fit of the piston and

sleeve. If the fit is tight, the piston will feel very
tight at top of the sleeve, and the engine will
have good sealing and compression. If the fit

is loose, compression will be low and both the
piston and sleeve should be replaced.

Flame-out - Occurs when the engine stops

running at high RPM. Usually the fault of
an excessively lean fuel mixture or glow
plug failure.

Fuel - (10%, 20%) The TRX 2.5R must have

model engine fuel to run. Traxxas Top Fuel

™

is recommended. Fuel is sold in quarts and
gallons from hobby dealers. The 10% and
20% labeling refers to the percentage
amount of nitromethane contained in
the fuel (see “nitro percentage” definition).

Fuel mixture - The ratio of fuel to air as

determined by the needle settings of
the carburetor.

Fuel tubing (fuel line) - The thick silicone

tubing that carries fuel from the fuel tank
to the carburetor.

Glow plug - The glow plug is located in the

cylinder head at the top of the combustion
chamber. It contains a platinum element that
glows red hot when voltage is applied. When
the engine is being started, the heat from the
glow plug ignites the fuel mixture and starts
the combustion process.

Glow plug driver - This tool clips onto the glow

plug and supplies the required voltage to
light the glow plug element. It is also called

TRX 2.5R •9

TERMS TO KNOW

an igniter. EZ-Start equipped engines do not
require this separate tool.

Header - The aluminum tube that connects

the exhaust system to the engine exhaust
port. The length and diameter of the header
must be carefully selected to extract the
most power from the engine.

High-speed needle (HSN) - Adjusts the

carburetors fuel/air mixture at high RPM.

Idle speed - The speed (RPM) the engine

runs at when the transmitter’s throttle
trigger is at neutral.

Idle speed screw (ISS) - Located on the

carburetor body. This screw adjusts the idle
RPM of the engine.

Lean - A running condition where the

engine is not getting enough fuel (for the
available air). Symptoms include engine
overheating, or the engine runs for a short
time and then stalls, particularly at high
speed. This is a dangerous condition that
should be corrected immediately or it can
ruin your engine.

Leaning the mixture - Turning either the

high speed and/or low-speed needle(s)
clockwise to decrease the amount of fuel
the engine receives.

Low-speed needle (LSN) - Needle valve that

controls the low-speed fuel mixture.

Needle valve - Valve consisting of a tapered

needle that closes against a corresponding
seat to regulate fuel flow.

Nitro - Abbreviation for nitromethane,

a component of model engine fuel that
improves fuel combustion and power
output. Nitro also refers to a class of
R/C powered by model engines instead
of electric.

Nitro content - The amount of nitromethane

used in the fuel. Usually measured as a
percentage of the total fuel volume. Traxxas
engines are optimized to use 10-20% nitro.
30% nitro may be used for racing.

Nitromethane - Nitromethane is a component

in the fuel that increases power from the
combustion process up to a point. Engines
are generally optimized to use a range of
nitro content for the best power.

O-ring - Rubber “O”-shaped ring used as a

sealing gasket.

Pipe - Abbreviation for the tuned exhaust pipe

on a nitro engine. See “Tuned Pipe”.

Piston - The piston is the internal engine part

that is attached to the upper end of the
connecting rod and moves up and down in
the cylinder sleeve. The precise fit between
the piston and the sleeve creates a seal that
allows engine to have the required
compression for combustion.

TRX 2.5R •10

TERMS TO KNOW

Port - Ports are openings in the sleeve that

allow atomized fuel to enter the combustion
chamber and burned exhaust gasses to exit.
The shape and location of the ports are a
large factor in controlling the engine timing
and power output.

Priming - Manually causing fuel to move from

the fuel tank up to the carburetor. This is
sometimes necessary after the engine has
been sitting for a long period of time and all
of the fuel has drained back to the tank. On a
Traxxas model this is done by holding your
finger over the exhaust tip for one or two
seconds while the engine is starting.

Punch - A term that refers to how quickly the

model responds to throttle input or how
quickly it accelerates.

Rich - A running condition where the engine is

getting too much fuel for the available air. It
is better to run an engine slightly rich to
increase engine life. Excessively rich mixtures
cause the engine to have sluggish
performance with exaggerated blue smoke
and unburned fuel coming from the exhaust.

RPM - Abbreviation for revolutions per minute

(how many times the engine crankshaft
spins in a minute).

Sleeve - Internal engine part that contains

the piston. The precise fit between the
sleeve and the piston creates a seal that

allows engine to have the required
compression for combustion. The sleeve
in a TRX engine is made of brass and is
then hard-chrome plated.

Slide carburetor - The throttle on a slide

carburetor closes and opens by sliding a
barrel in and out of the carburetor body.
This type of carburetor is preferred for
performance use because it provides a
less restrictive “straight-through” air path
than the barrel carburetor design.

Stall - When the engine stops running, usually

due to an incorrect fuel mixture setting or
running out of fuel.

Tuned pipe - The tuned exhaust pipe usually

consists of a specially-shaped metal or
composite chamber with baffles that is
designed to enhance the power output of
the engine.

Wear-in - Fitment process that occurs during

engine break-in where internal engine parts
develop an even more precise matched fit
through actual use under controlled
circumstances.

WOT - Abbreviation for wide-open throttle.

TRX 2.5R •11

THE FUEL

The Fuel

Use the Right Fuel

It’s imperative that you use the correct fuel in your TRX 2.5R for maximum performance and
engine life. Traxxas Top Fuel™should be used to ensure correct engine lubrication,
performance, and ease of tuning. Traxxas Top Fuel has been proven in thousands of engines,
so you can count on it every day for great performance.

• Top Fuel is the only fuel which is 100% certified to meet all requirements for use
in Traxxas engines.

• Traxxas Top Fuel is made with just the right balance of the highest grade natural and
synthetic lubricants to allow excellent throttle response and the best top-end performance,
without sacrificing long-term durability.

• All of the components in the fuel are carefully selected from the best materials available
and then custom blended to match the metallurgy and temperature characteristics of
Traxxas engines.

You may use 10% or 20% nitro content fuel. Use the same percentage all the time; don’t switch
back and forth between 10% and 20% fuel. If you break-in your engine on 20% fuel, then
always use 20% fuel.

What About Other Fuels?

Can other brands of fuel be used besides Top Fuel? There are other fuels that can provide
satisfactory performance, however, there could be long-term costs in the form of decreased
engine performance, loss of tuning ease, and shorter engine life.

Everyone has an opinion or a claim to make about fuel. The engineering team at Traxxas spent
over two years developing the TRX 2.5R Racing Engine. No one knows more about the specific
fuel requirements of Traxxas engines than Traxxas engineers. We strongly urge you not to take
chances with your engine investment and use the Traxxas fuel made for the TRX 2.5R.

You may use higher nitro
percentages than 20% in
certain circumstances.
Call 1-888-TRAXXAS for
more information.

TRX 2.5R •12

THE FUEL

Handling the Fuel

• Follow all directions and warnings on the fuel can.

• Keep the fuel tightly capped at all times. Some components in the fuel can evaporate very
quickly and upset the balance of the fuel.

• Do not store unused fuel in the fuel dispenser. Immediately return fresh unused fuel back into
the fuel can. Do not reclaim fuel from your vehicle’s tank.

• Do not mix old and new fuel. Never mix different brands of fuel together.

• Store the fuel in a cool dry location, away from any source of heat, ignition, or combustion.

• Read and follow the safety precautions on page 4 in this manual.

The Air Filter

The TRX 2.5R air filter is carefully designed to deliver maximum performance while protecting
your engine from dust and dirt. Use only the supplied filter. You will not improve engine

performance by switching to an aftermarket filter

, and you may risk engine damage due to

poor filtration.

The TRX 2.5R air filter assembly consists of 3 pieces:

1. A rubber filter base

2. A plastic housing consisting of a permanently assembled mesh cap and body

3. An oiled foam element

You must clean the filter after every hour of runtime, even if the filter looks clean. Dust (which
is often too fine to see) and dirt constantly move through the filter anytime the engine is
running. Even if you can’t see dirt on the filter, it is present inside the foam after any amount of
runtime. If you exceed the recommended cleaning intervals, your engine will be damaged.
Engine damage or wear due to dirt ingestion is easy to detect, and one of the top causes of
premature engine failure.

TRX 2.5R •13

THE AIR FILTER

Air Filter Cleaning Instructions

1. Remove the air filter from the carburetor bore by pulling the

entire filter assembly firmly to the side to release it.

Do not

pull straight up.

2.

Disassemble the filter. Pull the rubber air filter neck out of
the filter body. With the base removed, the foam element is
visible in the bottom of the filter body. Pull out the foam
element.

3. Clean the filter parts by thoroughly washing all three pieces of the filter assembly in hot

soapy water (dishwashing detergent works well). Repeat twice.

4. Thoroughly dry the parts with a clean towel or compressed air. Remember to wear your

safety glasses when working with compressed air.

5. Oil the foam element with foam filter oil. Use the supplied Traxxas filter oil (part# 5263) or a

high-quality, special-purpose foam filter oil like what is used for off-road motorcycle and
ATV engines.This type of filter oil is available at motorcycle pro shops. Apply 30 drops of the
Traxxas filter oil evenly to the top, bottom and sides of the filter element (30 total drops
divided among the 3 surfaces). Squeeze the filter element repeatedly to help spread the oil
throughout. The filter element should be evenly colored by the oil. Even color indicates that
the oil is distributed evenly.

Do not squeeze out excess oil.

Note: Do not use the air filter oil for anything other than the air filter. It is not meant
to be a lubricant.

6.

Reassemble the filter and install it on the engine, making sure the rubber filter neck fits
securely on the carburetor with no gaps or air leaks.

Clean your air filter after every
hour of runtime to avoid
engine damage, even if the
filter looks clean.

If you use compressed air to
dry your air filter assembly,
wear safety glasses and avoid
directing compressed air
toward your skin.

If foam filter oil is not
available, clean 30 or 40
weight motor oil can be
substituted, but is not
preferred. Never run the
engine without a properly
oiled filter!

TRX 2.5R •14

THE CARBURETOR

The Carburetor

Understanding the carburetor adjustments

The carburetor performs several functions. It
controls the engine’s speed by restricting the
intake of air and fuel into the engine. It atomizes
the fuel (suspends the fuel droplets in the air) and
also controls the air/fuel ratio of the mixture
entering the engine (how much air for a given
amount of fuel). To help provide a better
understanding of engine tuning and why it’s necessary, the
following is a brief explanation of the air/fuel combustion process
that takes place inside the engine.

In order to create the cylinder pressure that results in power, the engine burns the air/fuel
mixture. Both air and fuel, in correct amounts, are needed for proper combustion. It is the
carburetor’s job to mix the air and fuel together (atomize the fuel), in the correct proportion for
the best possible combustion. This is the ideal air/fuel ratio required for the engine. The ideal
air/fuel ratio remains roughly constant. Due to variations in atmospheric conditions
(temperature, humidity, altitude, etc.) fuel flow adjustment valves (called fuel mixture needles)
are required to meter the fuel and maintain the ideal air/fuel ratio in these ever-changing
conditions. For example, colder air is more
dense (more air molecules) for a given
volume of air and therefore requires more
fuel (more fuel molecules) to maintain the
correct air/fuel ratio. Warmer air is less
dense (fewer air molecules) and therefore
needs less fuel to maintain the correct
air/fuel ratio. The tuning needles are there
to adjust how much fuel is made available
for the carburetor to mix with the available
air (atomization).

Air

Fuel

Dense Air Less Dense Air

Factory Needle Settings

If your factory preset
carburetor adjustments
have been tampered with,
use the following settings:

• Set the high-speed needle
to 4 turns out from closed.

• Set the low-speed needle
so the screw head (red in
Fig. A) is flush (even) with
the end of the slide (yellow
in Fig. A).

Always use the factory
settings for initial starting.
Only use these settings
when the factory settings
have been lost.

Low Speed

Needle Adjustment

Fig. A

TRX 2.5R •15

THE CARBURETOR

The Fuel Mixture Needles

The amount of fuel metered and atomized by
the carburetor is controlled by the two mixture
needles; the high-speed needle and the low­speed needle. The low-speed needle is used to
meter the fuel used by the engine at idle and
low RPM (part-throttle) engine speed. The high­speed needle is used to meter the fuel when the
throttle is open from part throttle to wide-open
throttle (WOT). Two needles on the TRX 2.5R
provide precise control of the air/fuel ratio across the engine’s entire RPM range.
The maximum possible fuel flow is always controlled by the high-speed needle. It works like the
main water valve on a garden hose. Turn it clockwise to close the valve, counter-clockwise to
open it. When the throttle is at idle or partially open, the low-speed needle again meters the
fuel flow at the outlet (needle seat) where the fuel enters the carburetor venturi. This second
valve acts like the spray nozzle at the end of the garden hose in our example. When you
accelerate from idle, the throttle opens and the low-speed needle is pulled away from the
needle seat. This allows more fuel to flow with the increased air flow. As the throttle is
increased, the low-speed needle is pulled completely away from the needle seat leaving it fully
open. At that point, fuel metering is entirely controlled by the high-speed needle. Again, using
our water hose example, when the spray nozzle at the end of our garden hose is fully open,
then the main water valve can be used to adjust how fast the water flows.

The engine’s performance is directly linked to the fuel mixture. Richening the fuel mixture
increases the amount of fuel in the air/fuel mixture ratio and leaning the fuel mixture
decreases the amount of fuel in the air/fuel mixture ratio.

• Slightly rich fuel mixtures deliver cooler
running and more lubrication but with
slightly less power.

Water Valve

(Controls Overall Flow)

High Speed Needle

Water Nozzle

(Controls Fine Flow)

Low Speed Needle

• Slightly lean fuel mixtures deliver stronger,
more efficient combustion and more power,
but with less lubrication.

See Tuning the TRX 2.5R for
Best Performance on page

22 for complete information
on adjusting the air/fuel
mixture and idle speed.

TRX 2.5R •16

THE CARBURETOR

Tuning the engine means finding the
perfect balance between the two;
excellent power to meet your needs
while maintaining good lubrication
for long engine life.

The optimal fuel

mixture setting is rich to provide a

safety margin against having a lean
condition if some variable changes
(such as the temperature from one
day to the next). General fuel mixture
settings are measured by the number of turns the needles are turned out from fully closed. The
fuel mixture settings have been pre-set from the factory to typical break-in settings. Do not
readjust your carburetor from the factory settings until after the engine is started and running,
and you have been able to observe the engine running to assess what minor adjustments may
be required to compensate for fuel, temperature, and altitude. Adjustments are usually made in
1/8 or 1/16-turn increments.

The Idle Speed Adjustment

The idle speed screw controls the closed position of the throttle slide. When the throttle servo
is in its neutral position, the throttle slide should be stopped against the idle adjustment screw.
Always use the idle speed adjusting screw to control engine idle. Do not use the throttle trim
on the transmitter to adjust idle speed. The idle speed should be set as low as possible and still
maintain reliable running.

Lean

Rich

TRX 2.5R •17

BREAKING-IN THE TRX 2.5R

Breaking in Your TRX 2.5R Racing Engine

The TRX 2.5R Racing Engine uses a ringless, aluminum-brass-chrome (ABC) piston/sleeve
construction. This type of engine design relies on a very precise running fit between the piston
and sleeve for cylinder sealing.

Engine break-in is necessary to allow the piston and sleeve to
develop an extremely precise fit and optimum cylinder sealing. Therefore, proper engine
break-in is critical to achieving the fastest, most reliable engine performance.

Allow yourself about 1 to 11/2hours to complete the break-in procedure. The engine break-in
period will take 5 tanks of fuel. The break-in time is not the time to impress your friends with
your new engine.

You must wait until the engine is fully broken in before attempting

sustained high speed running.

Patience and careful attention during break-in will reward you

with the best-performing TRX 2.5R Racing Engine possible.

During break-in, your engine may appear to malfunction with symptoms like stalling,
inconsistent performance, and fouled glow plugs. These are simply the normal “break-in pains”
engines sometimes go through. They will disappear once your engine is fully broken in. Many
owners report not experiencing any of these symptoms with the TRX 2.5R Racing Engine.

We

recommend replacing the glow plug with a new one after the engine break in procedure.

Engine Break-in Procedure

The focus during break-in is to vary and limit the engine speed. This will be accomplished by
accelerating and stopping at different rates for the first 5 tanks of fuel. As the engine begins to
break-in, the duration and intensity of the acceleration will gradually increase.

Sustained high-

speed running is not permitted until the 6th tank of fuel.

Perform the initial break-in on a

large, flat, paved surface.

Apply all throttle and braking actions gently. Abrupt acceleration

or braking could cause the engine to stall unnecessarily.

Never run your model
indoors. Since the TRX 2.5R
Racing Engine exhaust
fumes are harmful, always
run your model outdoors,
in a well-ventilated area.

Your TRX 2.5R Racing
Engine doesn’t usually
require priming. If you do
need to prime your
engine, watch the fuel line
carefully to avoid flooding
your engine. See page 29
for information on clearing
a flooded engine.

Your carburetor is pre-set
at the factory to give the
correct air-to-fuel ratio
and idle speed for engine
break-in. Do not adjust
the carburetor unless
you observe a poor
running condition that
requires correction.

TRX 2.5R •18

BREAKING-IN THE TRX 2.5R

Important points to remember during break-in:

• Special break-in fuels are not recommended. Use the same fuel you plan to use everyday.

• If possible, avoid breaking-in the engine on extremely hot or cold days.

• Pay careful attention to the fuel level. Do not allow the fuel tank to run completely empty. An
extremely low fuel level causes the mixture to run too lean. This could result in a burned glow
plug or extremely high engine temperatures.

•

Do not attempt to break in the TRX 2.5R Racing Engine by idling it on a stand. This will produce
poor results.

• Keep extra Traxxas glow plugs (part #3231 or 3232) handy. The break-in process can cause
deposits to form on the plug leading to plug failure.

• Change or clean your air filter after break-in.

• Follow the instructions exactly for each of the first 5 tanks of fuel.

Starting Your TRX 2.5R Racing Engine for the First Time

Before you start your TRX 2.5R Racing Engine for the first time, make sure you have read all
instructions and precautions in this manual.

Pay close attention to the tank-by-tank break-in
instructions in the next section, and make sure you have read and understood them before
you run your engine.

Your engine must be at room temperature (70°F or 21° C) or above the first time you start it

.

If it’s cooler than room temperature outside, keep your TRX 2.5R Racing

Engine indoors until you’re ready to start it, then take it outside.

1. Turn on the radio system .

2. Make sure the throttle is in the idle position.

3. Pull the starter handle with quick, short, continuous pulls, and watch for

fuel moving through the fuel line up to the carburetor. Watch closely!
The fuel moves very fast. If the fuel doesn’t move through the line within
5 seconds, prime the engine by briefly (one or two seconds) covering the
exhaust outlet until the fuel is just visible in the carburetor fuel line.

Watch carefully! If the engine is primed too long, it will flood with fuel and stop turning.

See Important Points to
Remember on page 4 for
other precautions.

TRX 2.5R •19

BREAKING-IN THE TRX 2.5R

5. Once fuel reaches the carburetor, the engine should quickly start and idle.

6. Proceed with the engine break-in.

If your engine doesn’t start, refer to the TRX 2.5R Racing Engine troubleshooting section
at www.Traxxas.com. If you still have problems, contact Traxxas Customer Support at
1-888-TRAXXAS.

Tank 1

1.

Drive the model with the body off.

2.

Driving procedure: Gently pull the throttle trigger to about 1/4 throttle over a 2-second
count. Then gently apply the brake to stop

. Count the two seconds out while accelerating:
one thousand one, one thousand two, and then stop. Operate the throttle trigger as
smoothly as you can.

Repeat this starting and stopping procedure until the first tank

of fuel is nearly empty.

3.

Look for thick blue smoke exiting the exhaust outlet. If there is no smoke, richen the
high-speed needle 1/4 turn, by turning the needle counterclockwise.

4. When the fuel tank is nearly empty, shut off the engine by pinching the fuel line connected

to the carburetor (use the installed clamp).

5. Let the engine cool for 15 minutes.
Note: If at any point the engine cuts out or stalls during gentle acceleration, richen the high-speed
needle 1/4 turn by turning the needle counterclockwise.

Tank 2

1.

From tank 2 forward, model should be driven with the body on.

2.

Driving procedure: Gently pull the throttle trigger to about 1/2 throttle over a 2-second
count. Then gently apply the brake to stop.

Count the two seconds out while accelerating:

one thousand one, one thousand two, and then stop.

Repeat this starting and stopping

procedure until the second tank of fuel is nearly empty.

3.

When the fuel tank is nearly empty, shut off the engine and let it cool for 15 minutes.

As you gain experience in
the hobby, you may
discover that many people
have differing opinions on
what is the proper
procedure to break-in a
model engine.

Only use the
Traxxas break-in procedure.
Other break-in procedures
could result in a weak,
damaged, or otherwise poor
performing engine.

The
procedure outlined here
was extensively tested and
proven to yield better
performing engines than
other “common” break-in
methods. Even if you have
years of experience using
model engines, please do
not ignore this caution!

TRX 2.5R •20

BREAKING-IN THE TRX 2.5R

Tank 3

1.

Driving procedure: Gently pull the throttle trigger to 1/2 throttle over a 3-second count.
Then gently apply the brake to stop.

Count the three seconds out while accelerating: one

thousand one, one thousand two, one thousand three, and then stop.

Repeat this starting and

stopping procedure until the third tank of fuel is nearly empty.

2.

As the engine loosens, the idle speed may increase and cause the model to try to creep
forward when stopped. Reduce the idle speed
by turning the idle adjustment (see page 19) on the carburetor counterclockwise.

3. When the fuel tank is nearly empty, shut off the engine and refuel. From here on, you do not

need to let the engine cool between tanks.

Tank 4

1.

Driving procedure: Gently pull the throttle trigger to full throttle over a 3-second count. Then

gently apply the brake to stop.

Count the three seconds out while accelerating: one thousand

one, one thousand two, one thousand three, and then stop.

Repeat this starting and stopping

procedure until the fourth tank of fuel is nearly empty.

2.

Apply the throttle gradually! Your finger should not reach full throttle until the end of the
three-second count.

3. Keep your driving smooth and consistent.

4. When the fuel tank is nearly empty, shut off the engine and refuel.

Tank 5

1.

Driving procedure: Gently pull the throttle trigger to full throttle over a 3-second count, hold
for 2 more seconds, and then gently apply the brake to stop.

Count the five seconds out

while accelerating.

Repeat this starting and stopping procedure until the fifth tank of fuel is

nearly empty.

2.

The model should now be shifting into second gear. If it is not, try turning the high-speed
needle clockwise 1/8 turn to lean the fuel mixture slightly and test for shifting.

3. When the fuel tank is nearly empty, shut off the engine and refuel.

Tank 6

STOP! Clean your air filter before you proceed. Refer to the instructions on page 13. During the

sixth tank of fuel, the engine can be tuned for general performance use. Proceed to the next
section in this manual for instruction on tuning the engine.

Winter Break-in Tips

During the break-in process, the piston and sleeve wear into each other to form a precise fit. The
engine needs to heat up to a temperature of at least 200 to 215 degrees to allow the piston and
sleeve to achieve this fit properly. A precise fit between these two components is critical for
proper compression and optimum performance. If the engine runs too cold during break-in, the
piston and sleeve will not expand to their appropriate sizes for break-in. This can lead to
premature wear of these components. This wear may not become apparent until winter
has passed and the engine is operated under warmer running conditions.

• Warm the engine to approximately room temperature by removing all fuel and storing the
vehicle inside at room temperature until just before starting the engine. An extremely cold
engine can become difficult to start.

• After the engine is running, it is important to keep the temperature of the engine to at least 200 to
215 degrees during break-in. In weather below 45 degrees, the TRX 2.5R Racing Engine will tend to
run at lower temperatures between 160 to 180 degrees (when tuned at proper break-in mixture
settings). This is too cool for break-in.

Do not lean the engine to increase engine temperature!

This will also decrease lubrication and cause your piston/sleeve to wear prematurely.

TRX 2.5R •21

BREAKING-IN THE TRX 2.5R

Tank

1
2
3
4
5

Throttle

1/4
1/2

1/2
Full
Full

Time

2 Seconds
2 Seconds
3 Seconds
3 Seconds
5 Seconds

Cool

15 Minutes
15 Minutes

-

-

-

Body

Off
On
On
On
On

Notes

Apply throttle gradually.
Apply throttle gradually.
Reduce idle speed if necessary.
Gradually accelerate over 3 second count.
Accelerate over 3 second count,

hold for 2 seconds.

TRX 2.5R •22

TUNING THE TRX 2.5R

• Wrap the cooling head with a paper towel, clean rag or sock to help keep the engine running at
the recommended 200 to 215 break-in temp. If too much heat is contained, the engine may run
too hot. Make sure that you monitor the engine's temp closely for the first couple of tanks until
you get the right amount of cover for the cooling head. This will, of course, depend on your
current weather conditions. Adjusting the cover up and down, exposing more or fewer cooling
fins, is a convenient way to regulate engine temps.

• For owners that do not have access to a temperature probe, a drop of water on the cooling head
(around the glow plug area) should slowly sizzle for approximately 6 to 8 seconds when the
engine temp is 200 to 210 degrees. If the water sizzles for only a few seconds, then it is likely that
the engine temp is over 220 degrees and needs to cool down. If the water takes a long time or
does not evaporate at all, then the engine is too cool.

• We do not recommend that you operate your engine below 35 degrees Fahrenheit. If you insist
on running your vehicle below 35 degrees, be aware that nitro engines may be very difficult to
start and tune at those extremely cold temperatures. Also, at temperatures below freezing, nitro
fuel can actually begin to gel up, and this can be harmful to the engine.

• Follow the remaining break-in procedures as outlined in this owner's manual. This, along with the
steps listed above, will ensure a good break-in for your new nitro engine and provide many hours
of enjoyment.

Tuning Your TRX 2.5R Racing Engine

The engine’s performance depends on the fuel mixture. Turn the mixture needles clockwise to
lean the fuel mixture and counterclockwise to richen it. Leaning the fuel mixture will increase
engine power up to the engine’s mechanical limits.

Never run the engine too lean (not
enough fuel flow). Never lean the engine until it begins to cut-out or stall. Leaning the

engine beyond the safe allowable limits will result in poor performance and almost certain
engine damage.

Indications of an overly lean mixture include:

•

Cutting out or sudden loss of power during acceleration.

• Overheating (temperature beyond 270° F at the glow plug)

• Little or no blue smoke coming from the exhaust.

TRX 2.5R •23

TUNING THE TRX 2.5R

If any of these conditions are present, stop
immediately and richen the high speed mixture
1/4 turn. The engine will probably be slightly
rich at that setting and you can then retune for
performance. Always tune for performance by
starting rich and moving leaner toward the ideal
setting.

Never try to tune from the lean side.
There should always be a light stream of blue
smoke coming from the exhaust.

Before you
begin tuning, the engine should be warmed up
to its normal operating temperature and
running slightly rich. All final tuning adjustments
must be made to the engine at its normal operating
temperature. You can tell the engine is running rich by
noting any of the following:

• Sluggish acceleration, with blue smoke coming from

the exhaust.

• There is unburned fuel spraying from the exhaust tip

• Leaning the high-speed fuel mixture

increases performance

High-Speed Fuel Mixture Adjustment

With the engine warm and running at a rich setting, gradually lean the high-speed fuel mixture
in 1/16 turn increments. Make several high-speed passes after each adjustment to clear out the
engine and note any change in performance. The TRX 2.5R is extremely powerful. Remember to
apply the throttle gradually to prevent wheelies or loss of control.

Continue this procedure until

there is no longer any performance improvement.

If any one of the following conditions occurs,

the fuel mixture is already past the maximum safe lean setting:

1.

There is no longer any performance improvement

2. The engine begins to cut out at high speed (Danger!)

When tuning for
performance, watch closely
to notice when there is no
longer any increase in speed
or power when the high­speed mixture is lean.
If you lean the high-speed
mixture to the point that the
engine cuts-out, hesitates, or
stalls, you are well into the
danger zone and engine
damage is likely. Richen the
high-speed needle 1/4 turn
and retune.

TRX 2.5R •24

3. There is a sudden loss of power during acceleration (Danger!)

4.

The engine begins to overheat. Symptoms of overheating include:

If any of the above conditions occcur, richen the fuel mixture to the optimum setting by
richening the high-speed needle at least 1/8 turn counterclockwise and retest. This setting
will extend engine component life.

Low-Speed Fuel Mixture Adjustment

The low-speed mixture is always set after the high-speed needle is correctly adjusted.
Set the low-speed mixture using the following pinch test.

1. Once the engine is warm, do several high-speed runs to confirm that the high-speed needle

is set correctly.

2. Bring the vehicle in and pinch closed the fuel line going into the carburetor. The engine

should run for 2-3 seconds, speed up, and then die.

3.

If the engine runs longer than 3 seconds, then lean the low-speed needle 1/16 turn,
make several more high-speed runs, and retest.

4. If the engine dies immediately without speeding up, then richen the low-speed needle 1/8

turn, make several more high-speed runs, and retest.

When the low-speed needle is set correctly, the engine’s throttle response should be very quick.

Idle Speed Adjustment

Once the high and low-speed mixtures have been set, reduce the idle speed to the minimum
reliable idle speed. Remember, this adjustment should be made while the engine is running at
normal operating temperature.

1. Turn the throttle trim on the transmitter so that the brakes are applied. This ensures that

the throttle slide is resting against the idle adjustment screw.

TUNING THE TRX 2.5R

For your convenience, the
low-speed needle has a
positive stop that prevents
it from being over tightened
and damaging the needle
and seat. This also provides
an easy way to gauge how
many turns out from closed
the low-speed needle
adjustment is set to.

• Fluctuating idle speed

• Hesitation or stalling during acceleration

• Steam or smoke coming from the engine
(not exhaust)

• Popping or clattering sound when
decelerating (detonation)

• Temperature measurement above 270° F
at the glow plug

When adjusting the low­speed fuel mixture, it is
very important to make
several high-speed runs
with your model between
adjustments to clear out
any excess fuel. Perform
the pinch test immediately
after. If the engine is
allowed to idle for a long
period of time, it could
“load up” with fuel and
give you an inaccurate
measurement from your
pinch test.

TRX 2.5R •25

2.

If necessary, remove the air filter to gain access to the idle adjustment screw.

3.

Turn the screw counterclockwise to reduce the idle speed, or clockwise to increase it.
The idle speed should be set as low as possible while still maintaining reliable
running characteristics.

4.

Reset the throttle trim on the transmitter to its original position.

Fine-Tuning the Carburetor

After fine-tuning your TRX 2.5R Racing Engine at the end of the break-in procedure, no major
adjustments to the fuel mixture are usually necessary. Make note of the temperature, humidity,
and barometric pressure at the time you finished fine tuning your carburetor. Current weather
conditions can be found online from national websites, local TV news websites, and television.
This information will be considered your baseline setting. You may need to adjust your
carburetor needles to compensate for changes in temperature and barometric pressure (air
density) from day to day. Generally, you’ll need to richen the fuel mixture when the weather is
colder than your baseline temperature and the air density is higher. Lean the fuel mixture when
weather is warmer than your baseline temperature and the air density is lower. The chart below
provides general guidelines on how weather conditions affect air density when they move
higher or lower than your baseline setting (see page 14 for detailed info on how air density
affects mixture settings).

If the...

Humidity

Pressure (barometer)

Temperature

Altitude

Nitro %

is..

Lower
Higher
Lower
Higher
Lower
Higher
Lower
Higher
Lower
Higher

then the

air density is...

Slightly more dense
Slightly less dense

Less dense
More dense
More dense
Less dense
More dense
Less dense

adjust (correct)

the fuel mixture to be...

Slightly richer
Slightly leaner

Leaner
Richer
Richer
Leaner
Richer
Leaner
Leaner
Richer

TUNING THE TRX 2.5R

TRX 2.5R •26

TUNING THE TRX 2.5R

Tuning the Engine by Temperature

The following procedures require an optional infrared temperature probe or on-board temperature
gauge (Traxxas on-board digital temperature gauge, part #4091). Engine temperature can be used
as an effective tuning aid when you understand the relationship between engine temperature and
ambient temperature. The engine operating temperature, when tuned for maximum performance,
will vary according to atmospheric conditions. The atmospheric condition that has the most
influence on engine temperature is air temperature. Expect the engine temperature to vary almost
in direct proportion to air temperature. Assuming you tuned the engine for the same maximum
performance each day, the engine will run about twenty degrees hotter when it’s ninety degrees
outside than it would in seventy-degree weather. For this reason, we cannot give you a definitive
temperature range that indicates the best possible engine tuning.
The temperature gauge can aid you in tuning by giving you a relative indication of how your
adjustments are affecting the engine and by preventing you from exceeding maximum engine
temperature. For example, as you lean the fuel mixture, the engine performance will increase along
with the temperature. If you continue to lean the fuel mixture and the temperature increases but
the engine performance does not change, then you have exceeded the maximum safe lean
setting. Even if it’s 100°F outside, you should be able to tune for maximum performance. Try to
keep your engine from exceeding 270°F when measured at the glow plug. If tuning for maximum
performance results in engine temperature exceeding 270°F, try to increase airflow to the engine
by cutting out the rear of the body, windshield, and front valance. If the engine temperature still
cannot be kept below 270°F, richen the high-speed needle slightly.

TRX 2.5R •27

TROUBLESHOOTING THE TRX 2.5R

Troubleshooting Your TRX 2.5R Racing Engine

The following section addresses some very basic engine and radio questions you may have about
your TRX 2.5R Racing Engine. Most questions arise from simple user errors or minor adjustments that
are easy to correct. If you can’t find a solution for your TRX 2.5R Racing Engine here, visit our web site
at Traxxas.com and click on the Customer Support menu. There you will find a much more extensive
and detailed online troubleshooting area. In addition, you may call Traxxas Customer Service at
1-888-TRAXXAS (outside the US call 972-265-8000).

Engine spins but will not start:

•

If you’re using a glow plug igniter, make sure it’s fully charged and connecting properly to the glow
plug. If the igniter is fully charged, then replace the glow plug. It is normal for glow plugs to require
periodic replacement. Only use Traxxas heavy-duty long glow plugs (part #3231 or #3232).

•

Check to make sure the fuel is getting to the carburetor. Remove the fuel line where it connects to
the carburetor to see if there is fuel in it. If not, you may need to prime the engine. Reconnect the
fuel line and then refer to page 18 for instructions on priming the engine.

•

Check your fuel mixture settings. It may be necessary to adjust the fuel mixture if the outside
temperature or barometric pressure has changed significantly since the last time the engine
was run (see Fine Tuning the Carburetor on page 25). Turn the high-speed needle out
(rich/counterclockwise) 1/4 turn, hold the throttle trigger at about 1/2 throttle and try again
to start the engine. Once started, retune the engine for performance (see page 22).

•

The engine could be worn. If the fit between the piston and sleeve is loose, compression is reduced
and the engine will be difficult to start when it is warm, and may tend to stall when running and
when the throttle is closed suddenly to idle. Engine life depends on many factors including fuel
type, air filter maintenance, needle settings, and how the engine was used. For example, if the
engine was allowed to ingest dirt from lack of air filter maintenance or running through water, then
the internal engine components could wear out extremely fast.

TRX 2.5R •28

TROUBLESHOOTING THE TRX 2.5R

Engine will not spin (pull start or electric starter will not turn the engine):

•

The engine could be flooded. If too much fuel accumulates in the combustion chamber at start up, the
engine will hydraulically lock. Follow the procedure on page 29 for clearing a flooded engine.

•

Check for binding at the engine flywheel. If you are not able to turn the engine by hand, the engine
could be flooded, there could be binding in the clutch system, or there could be internal engine
damage. See page 29 for info on clearing a flooded engine and releasing a stuck piston. (Note: It is
normal for the engine to be extremely tight and hard to turn when the piston reaches the top of the
compression stroke.)

Engine extremely sluggish, hard to start, and will not idle during tank 1 of the break in:

•

It is possible that the factory adjusted break-in settings on your carburetor are too rich for your
geographic location, atmospheric conditions, or fuel brand. Extremes in temperature, humidity,
barometric pressure, and altitude can’t always be accounted for with a single high-speed needle
setting. The symptoms described above can occur when the air density is very low, such as in high
mountainous elevations and extremely cold temperatures. Under these conditions, lean the fuel
mixture slightly, 1/8 turn, and see if there is any improvement in starting and idling. Lean the fuel
mixture only until the engine will run and idle reliably, and then proceed with the break-in.

•

The contents of different brands of fuel (other than Traxxas Top Fuel) in combination with extreme
atmospheric conditions can also make the factory preset break-in settings too rich and cause the
symptoms described above. Again, try leaning the fuel mixture slightly, 1/8 turn, to see if there is
any improvement in starting and idling.

Engine performance sluggish:

•

Engine performance depends mostly on the fuel mixture settings and how they compensate for the
current atmospheric conditions. Before you suspect other failures with the engine, richen the high­speed needle at least 1/4 turn and then retune the engine for performance (see page 22).

•

Try a new Traxxas glow plug. Sometimes a glow plug will work well enough to start the engine but
not be able to deliver the engine’s full performance potential.

TRX 2.5R •29

TROUBLESHOOTING THE TRX 2.5R

•

If the fuel mixture seems to be set correctly, make sure the fuel is fresh. If the fuel is old, or was left
uncapped for a long period, then some of the important fuel components could have evaporated.
Try new, fresh, Traxxas Top Fuel.

•

Check to make sure there is no binding in the driveline that would cause excessive loads
on the engine.

Clearing a Flooded Engine:

If the engine is primed for too long during startup, it can become flooded with fuel. When the engine
is flooded it will no longer turn due to excess fuel in the combustion chamber, preventing upward
movement of the piston. Use the following procedure to clear a flooded engine:

1. Remove the glow plug and gasket with the glow plug wrench supplied with your model.
A 5/16" (8mm) nut driver will also work.

2. Turn the model upside down.

3. Pull the starter handle several times to clear the engine of excess fuel.

4. Turn the model over and reinstall the glow plug and gasket.

5. Do not prime the engine. Apply 1/2 throttle and pull the starter. The engine should
start immediately.

Piston Stuck at Top Dead Center (TDC):

A new engine will typically have a tight fit between the piston and the top of the sleeve. This is a
tapered fit. The fit should not be too tight to start the engine. If the engine is stuck at TDC, then use
a pair of needle nose pliers to grasp the flywheel from the bottom of the engine. Rotate the flywheel
counter-clockwise (when viewed from the air intake side [front] of the engine). You should see the
flywheel turn and feel the piston become unstuck from the sleeve.

TRX 2.5R •30

MAINTAINING THE TRX 2.5R

Maintaining and Storing Your TRX 2.5R Racing Engine

TRX 2.5R Racing Engine requires timely maintenance in order to stay in top running condition.

Neglecting the maintenance could allow dirt, deposits, and moisture to build up inside
the engine, leading to internal engine failure. Follow proper maintenance and storage procedures

to avoid damage to your engine. The following procedures should be taken very seriously.

After each hour of running:

• Clean and re-oil the air filter. The instructions for this procedure are on page 13. We cannot stress
enough the value of cleaning your air filter at the scheduled intervals. Do not neglect your air

filter maintenance!

• Clean the outside of the engine of accumulated dirt, oil, and grime. Accumulated grime will
decrease the engine’s ability to cool itself.

After each running session:

• Perform after-run maintenance on the engine. This clears the engine of destructive moisture and
other corrosive deposits.

This is extremely important for the life of the engine. See page 31 for

after-run maintenance procedures.

•Inspect the vehicle for obvious damage or wear. Look for:

1. Loose or missing screws

2. Cracked, bent, or damaged parts

3. Cut or loose wiring

4. Cut or kinked fuel lines

5. Signs of fuel leakage

• Inspect the gears for wear, broken teeth, or debris lodged between the teeth.

After-run Procedure

You must perform after-run maintenance on your Traxxas engine whenever the model will be
stored for longer than a few hours. Taking the time to prepare your engine for storage will reward
you with longer engine life, easier starting, and better performance.

If the engine is worn or
damaged enough to
require replacing the
piston, sleeve, or
connecting rod, consider
exchanging your old
engine for a brand new
engine under the terms of
the Traxxas Lifetime Engine
Replacement Plan (ERP). It
could save you time and
effort. Details are in your
documentation package
or on Traxxas.com.

Don’t put the fuel from
your tank back into your
fuel jug. Dispose of it
properly, following city or
county regulations.

Always wear eye protection
when using compressed air
or spray cleaners and
lubricants.

TRX 2.5R •31

MAINTAINING THE TRX 2.5R

When a nitro engine is shut off, some excess unburned fuel remains in the engine. The methanol in
model engine fuel is hygroscopic, which means it easily attracts and absorbs moisture. This moisture
can cause rust and corrosion on the steel engine parts (crankshaft, bearings, wrist pin and starter
shaft) if the fuel is not removed from the engine. There are after-run oil products available from your
hobby dealer or you can use WD-40

™

, a common household lubricant. To ensure your TRX 2.5R

Racing Engine is protected from internal corrosion, use the following procedure:

1. Whenever possible, shut off the engine by pinching the fuel line closed. This allows most of the

excess fuel to be consumed by the engine. Be sure the throttle is in the idle position. You may
have to pinch the fuel line closed for several seconds before the engine stops.

2. Completely empty the fuel tank. Use your fuel-dispensing bottle to suck out the old fuel.

Do not mix the old fuel with your fresh fuel supply. If you leave fuel in the tank, transporting
or handling your model may cause fuel to run into the engine.

3. With the fuel tank empty and the throttle at the idle position, try to start the engine.

The engine will most likely start and run for a few seconds as it uses up any fuel remaining
in the engine and fuel lines.

4. Once the engine stops, clean the outside of the engine with compressed air or spray

motor cleaner. Once the engine is clean and dry, remove the glow plug power wire,
glow plug, and air filter.

5. Open the throttle fully and spray a one-second burst of WD-40™into the carburetor and

into the glow plug hole (Caution! Wear safety glasses to prevent spray from getting into

your eyes).

6. Place a rag or paper towel over the engine to catch any WD-40™that may come out the

carburetor or glow plug hole.

7. Pull the starter handle continuously for about 10 seconds.

8. Remove the rag or paper towel and repeat steps 5–7 two more times.

9. Clean and re-oil the air filter so it will be ready for use next time. See page 13 for air filter

maintenance instructions.

10. Replace the glow plug and reinstall the air filter.

The TRX 2.5R is designed to
be easily rebuilt. Critical
engine components such

as the crankcase, crankshaft,
and engine bearings are
made to extremely high
quality standards and should
under normal circumstances
outlast multiple sets of pistons,
sleeves, connecting rods,
and wrist pins (reciprocating
assemblies). It could be more
economical for you to continue
to use your good bearings
and crankshafts, and simply
replace the reciprocating
assembly as needed. Engine
assembly is not difficult and
replacing the reciprocating
assembly does not require

any special tools or skills.

Denatured alcohol (available
from home centers and paint
supply stores) in a spray
bottle is an extremely
effective cleaner. Be sure
to wear safety glasses and
gloves when working with
denatured alcohol.

TRX 2.5R •32

MAINTAINING THE TRX 2.5R

Other periodic maintenance:

• Piston/sleeve:

The life of the piston and sleeve will vary greatly with how the engine was
used and maintained. The piston and sleeve should be replaced when they no longer seal
effectively (loss of compression). Symptoms include the engine being difficult to start when
warm, stalling when warm, and stalling when throttle is suddenly closed to idle. Replace the
wrist pin and G-clip whenever the piston and sleeve are replaced. Follow the instructions for
the proper break-in procedure of a new TRX 2.5R Racing Engine whenever the piston and
sleeve are replaced

.

• Connecting rod:

The connecting rod should be replaced when the piston and sleeve are
replaced or after three gallons of fuel, whichever comes first. Also replace the piston wrist
pin and G-clip whenever the connecting rod is replaced. As with other internal engine
components, the connecting rod’s life depends on the engine’s usage and the quality and
frequency of the engine’s maintenance.

TRX 2.5R •33

These instructions are for the rebuilding
of the Traxxas TRX 2.5R engine once the
engine has been removed from the
chassis. These instructions were
performed on an engine equipped with
the EZ-Start electric starting system.
Engines equipped with pull starters or
backplates (without starter) are similar.
It is very important to clean the exterior
of the engine and EZ-Start system
thoroughly before disassembly. These
instructions show you how to replace the
entire internal reciprocating assembly.

Rebuilding the TRX 2.5R Racing Engine

The following instructions are for engines
equipped with IPS crankshafts using Traxxas
clutch components. For multi-shaft

™

crankshafts in non-Traxxas applications,
follow the manufacturer’s directions for
clutch installation removal.

2. Remove the 5.0mm E-clip from the
crankshaft (Nitro 4-Tec owners will need to
remove the 3x10CS screw) and slide the
5x8x0.5TW with the clutch bell off
of the crankshaft

(Nitro 4-Tec owners will

need to slide two 5x8x0.5TW off the pilot
shaft.)

Caution: It’s important to use eye
protection when removing the E-clip
from the shaft to prevent the clip from
causing an injury. Pull the clutch shoes

off of the flywheel pins.

Remove the flywheel nut from the
crankshaft using an 8mm socket (202)
and disconnect the flywheel from the
crankshaft by lightly tapping the back
of the flywheel with a rubber or
plastic mallet.

Place a small flat blade screwdriver into
the slot of the split cone washer and twist
slightly to loosen the split cone from the
crankshaft (203). Slide the split cone
washer off of the crankshaft. The engine
is now ready to be rebuilt (204).

1. Start by removing the exhaust pipe
and header from the engine by removing
the two 3x15CS screws (102 & 103).

Remove the three 3x12BCS screws
that secure the EZ-Start (or pull start)
assembly to the engine and separate
the drive unit from the engine (104).
If your engine is equipped with a
backplate, proceed to step 2.

101

102

103

104

201

203

204

202

TRX 2.5R •34

REBUILDING THE TRX 2.5R

3. Remove the glow plug from the
cooling head and discard the plug.
The glow plug should be replaced
with a new one anytime the engine
is rebuilt.

Remove the four 3x6CS (3x6BCS on
engines equipped with non-starter
backplate) screws from the backplate of
the engine and pull the backplate from
the crankcase (301).

Remove the cooling head from
the engine case by removing the five
3x12CS head bolts (302). Be careful
not to lose or damage the copper
head gasket. If the head gasket is
damaged, replace it with a new one.

4. Locate the bottom of the sleeve
through the rear opening of the
crankcase. Press the bottom of the
sleeve upward with a plastic or
wooden stick (401), this is to prevent
internal damage to the crankcase.
The sleeve should slide up and out
of the crankcase (402). (Note: If the
sleeve does not slide out of the
crankcase with reasonable effort,
slightly warm the crankcase with a
hair dryer or heat gun to about 130° F.
The sleeve should slide out easily at

this temperature. Use gloves to
protect against burns. ) Rotate the
crank to where the rod and piston are
located at TDC (top dead center)(top
of the piston’s stroke)(403).

Gently pull on the bottom of the
connecting rod with needle nose
pliers, sliding the rod off of the crank
pin (403). Be careful not to damage
the connecting rod if the rod is
being reused.

The rod and piston assembly will
exit the top of the crankcase (404).

5. Push the end of the pilot
shaft through the crankcase. The
crankshaft should exit the rear of
the crankcase (501). Loosen the 3.0NL
just below the base of the carburetor
(502). Slide the carburetor up and out
of the crankcase (503). Now the
crankcase is ready to be flushed
clean and inspected.

301

302

401

402

403

404

501

502

503

6. Flush the crankcase and bearings
out with out with denatured alcohol or
electric motor spray. Dry the crankcase
with compressed air (601).

Caution: Do not let the compressed air
spin the bearings and always use eye
protection when using compressed air.

Inspect the crankcase and bearings
for damage or missing parts. Make sure
that the bearing cages are not broken
and check for corrosion inside of the
crankcase. The bearings should feel
smooth and have little play. Make sure
the bearings spin freely and that there
are no particles or grit in them. Replace
the bearings if there are any signs of
damage. Inspect the exhaust O-ring
gasket and the backplate O-ring gasket
for tears (602). Replace these O-rings
if needed.

7. Apply a few drops of machine oil or
after-run oil to the bearings. Inspect the
crankshaft for corrosion and also check
for scratches or score marks around the
intake port of the crankshaft (see arrow).
Check the crank pin for wear (701). The
connecting rod should fit and rotate
smoothly on the crank with no play
between the rod and the crank pin. If
there is play between a NEW connecting
rod and the crank pin then the crank pin
is worn and the crankshaft will need to
be replaced.

Important: Do not reuse a damaged
or worn crankshaft. This will dramatically
shorten the life of your rebuilt engine.
If there are damaged or worn parts beyond
the piston, sleeve and connecting rod it may
be best to take advantage of the Traxxas
Engine Replacement Program.

8. Slide the crankshaft back through the
crank bearings until it stops. Inspect the
carburetor O-rings for damage and the
rubber throttle arm boot for tears (801 & 802).

Unscrew the hi-speed needle all the way
out of the needle housing (803) and flush out
the hi-speed needle housing and the
carburetor body with denatured alcohol or
electric motor spray. Use compressed air to
dry the components.

Remember: Always

wear safety glasses when using compressed
air to prevent injury. Reinstall the hi-speed
needle into the needle housing and return
the mixture setting back to the factory
recommended break-in setting (four turns
out from fully closed).

Carefully insert the carburetor into
the crankcase until fully seated. While
pressing the carburetor firmly into the
crankcase, tighten the pinch bolt (804).
This process ensures that the upper
O-ring forms a good seal between the
carburetor and the crankcase.

601

602

701

801

803

804

802

TRX 2.5R •35

REBUILDING THE TRX 2.5R

10. Lube the outside of the piston
with after-run oil and make sure the
crank is at BDC (bottom dead center),
then slowly lower the sleeve until It is
just above the piston. Rotate the crank
slowly to bring the piston into the
sleeve. If there is any resistance, stop!
Repeat until the piston moves
smoothly into the sleeve. Bring the
piston close to TDC, then push the
sleeve in fully. Important: The slot (in
the lip) at the back of the sleeve must
key into the pin at the back of the
crankcase opening (1001). Do not
install the cooling head until these two
items are keyed into each other.

Place the copper head gasket onto the
sleeve and fit the cooling head into the sleeve (1002). Note that there are
two slots notched through the cooling fins on each side of the cylinder
head. These slots are for access to the engine mounting screws. The five
head bolts should line up with the five holes on top of the engine case
and the access slots on the cooling head should line up with the slots
manufactured into the crankcase. Be careful not to damage the copper
head gasket (see picture 1002 for orientation).

While pressing the head squarely onto the sleeve, thread the head bolts
into the engine case just until they stop. Do not tighten down these bolts
individually. It is very important to attain even pressure around the cooling
head. Slowly tighten each screw, just a little at a time, in a crossing pattern
(see picture 1003) until each bolt becomes snug. Finish each bolt in the
same pattern with 9-11in•lbs. of torque.

9. Apply a drop of after-run oil in the
piston’s wrist pin bore, before installing
the wrist pin. Locate the oil hole (refer to
pic 901) in the connecting rod and the
skirt relief side of the new piston.

Insert the connecting rod into the piston
with the piston skirt relief and the oil
hole on the same side (901). Line up the
wrist pin bore in the rod with the wrist
pin bore in the piston and slide the wrist
pin through the larger opening in the
piston. Note that the closed end of the
wrist pin should be inserted first into the
opening. The orientation of the wrist pin
helps keep the wrist pin cool.

Install the wrist pin clip into the groove
of the piston’s wrist pin bore (902). This
will retain the wrist pin inside of the
piston . Caution: Do not allow the pliers

to scratch the piston, and make sure
that the clip expands into the groove
before moving on.

Lower the piston and connecting rod
assembly into the top of the engine case
with the oil hole of the connecting rod
facing toward the crankshaft. Apply a
drop of oil onto the crank pin before
installing the connecting rod onto the
crank pin. Slide the connecting rod over
the crank pin and rotate the crank pin to
the bottom center of the crankcase (903).

901

902

903

1001

1002

1003

Skirt

Relief

Oil

Hole

TRX 2.5R •36

REBUILDING THE TRX 2.5R

Oil

Hole

Skirt Relief

11. Inspect the backplate bushing
and starter shaft for wear and clean
them with denatured alcohol. Fasten
the backplate with the starter shaft to
the crankcase.

Important: The reliefs

that are manufactured into the
backplate must line up to match the
ports inside of the engine case (see
picture 1101 for orientation).

Key the starter shaft to the crank pin
by slowly rotating the starter shaft
as pressure is applied towards the
crankshaft. Once the starter shaft is
keyed into the crank pin, join the
backplate to the engine case and
secure the assembly with the four
3x6CS screws (1102). (On non-starter
models simply fasten the backplate to
the crankcase with (4) 3x6BCS screws).

Install a new glow plug (Traxxas #3232
is recommended for the TRX 2.5R) into
the cooling head. The engine is now
rebuilt and ready for the installation
of the clutch assembly and the
EZ-Start system (if so equipped).

12. Slide the split cone washer over the
pilot shaft with the larger end of the
taper toward the engine case (1201).
Slide the flywheel over the pilot shaft
and onto the split cone washer.

The split cone washer should fit inside
of the tapered hole in the flywheel.
Thread the flywheel nut all the way up
to the flywheel and tighten the flywheel
with 55-65 in•lbs. of torque against the
flywheel (1202).

13. Slide the clutch shoes onto the
flywheel. Key the flywheel pins into the
holes of the clutch shoes (See picture
1301 for orientation of the clutch shoes).

Tip: By reversing the direction of the
clutch shoes, the clutch shoes will
provide a little firmer bite against the
interior wall of the clutch bell. This is
useful on higher bite surfaces that offer
a higher level of traction.

14. Clean and lube both clutch bell
bearings and install a ball bearing into
each side the clutch bell and slide the
clutch bell over the pilot shaft followed
by the 5x8x0.5TW (1401).

Fasten the 5.0 E-clip (wearing safety
glasses) around the pilot shaft to retain
the clutch assembly.

Note: The Nitro 4-Tec will need two
5x8x0.5TW placed onto the pilot shaft
before the installation of the clutch
bell, then secure the assembly with
the 3x10CS screw.

1201

1202

1203

1301

1401

1102

1101

TRX 2.5R •37

REBUILDING THE TRX 2.5R

15. Clean the roller clutch with denatured
alcohol or electric motor spray, and lube
the roller clutch with after-run oil. Slide
the roller clutch onto the starter shaft as
shown (1501).

Note: The roller clutch is to be installed
onto the starter shaft so that the clutch
engages the starter shaft clockwise and
rotates freely counter-clockwise. The
engraved text on the roller clutch faces
the engine backplate for correct rotation.

Slide the EZ-Start system or pull start onto
the starter shaft that exits the backplate of
the engine. Line up the drive with the
roller clutch. Fasten the EZ-Start gear case
to the backplate with the three 3x12BCS
screws (1502). Fasten the pull starter with
(4) 3x12BCS.

16. Inspect the exhaust O-ring and
replace it if it is damaged or torn. Fasten
the exhaust header and pipe assembly
back onto the rear of the engine case with
the two 3x15CS screws (1601).

The engine, clutch, exhaust system
and starter assembly is now ready
for installation into the chassis (1602).

Follow the instructions for the proper
break-in procedure of a new TRX 2.5R
Racing Engine.

1501

1502

1602

1601

TRX 2.5R •38

REBUILDING THE TRX 2.5R

TRX 2.5R •39

INDEX

.15; 7
ABC; 7, 17
After-run Procedure; 30, 31
Air Filter; 5, 6, 7, 8, 12, 13

aftermarket filter; 12
cleaning; 13
filter foam; 8
oil; 13

Air Intake; 6
Backplate; 6
Big Block .15; 7
Break-in; 7, 17, 18, 19, 20, 21

Tank 1; 19
Tank 2; 19
Tank 3; 20
Tank 4; 20
Tank 5; 20
Tank 6; 21
winter break-in; 25

Carburetor; 6, 7, 14, 15, 16

carb; 7
factory settings; 15
slide; 10
understanding; 14

Clean-out; 7
Clutch Bell; 6
Combustion chamber; 7
Connecting rod; 7
Cooling fins; 8

Crankcase; 6, 7
Crankshaft; 6, 7
Cylinder head; 8

head protector; 6
PowerTune™head; 6

Dyno; 8
EZ-Start; 8
Fit; 8
Flame-out; 8
Flywheel; 6
Fuel; 4, 8, 11

handling; 12
intake; 6
nitro content; 11
other fuels; 11
Traxxas Top Fuel; 11
tubing; 8

Fuel Mixture; 8, 15

atomization; 14
high-speed mixture; see “High­Speed Adjustment”
lean; 9, 15, 16
leaning; 9
low-speed mixture; see “Low-

Speed Adjustment”
ratio; 14
rich; 10, 15, 16

Glow plug; 8

driver; 9

Header; 9
High-Speed Adjustment; 22

high-speed needle; 6, 9

Idle Speed; 9, 16, 23

idle speed screw; 6, 9

Low-Speed Adjustment; 23

low-speed needle; 6, 9

Maintaining and Storing; 30
Needle valve; 9
Nitro; 9
O-ring; 9
Piston; 9
Port; 10
Priming; 10
Pull Starter; 6
Safety; 4
Sleeve; 10
Stall; 10
Starting; 18
Throttle Arm; 6
Troubleshooting; 27

clearing a flood; 29

Tuned pipe; 10
Tuning; 21

by temperature; 26
fine-tuning; 24

Wear-in; 10
WOT; 10

1100 Klein Road, Plano, Texas 75074

1-888-TRAXXAS

®

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