Kymco MO GD 250 Service Manual - chap13 (alimentazione)

13. FUEL SYSTEM/CARBURETOR/

FUEL PUMP/ FUEL TANK

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FUEL SYSTEM/CARBURETOR/FUEL PUMP

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FUEL SYSTEM --------------------------------------------------------- 13- 1
SCHEMATIC DRAWING ---------------------------------------------- 13- 2
OPERATION OF CARBURETOR JETS------------------------------ 13- 3
SERVICE INFORMATION -------------------------------------------- 13- 5
CARBURETOR REMOVAL ------------------------------------------- 13- 7
VACUUM CHAMBER DISASSEMBLY------------------------------ 13- 7
FLOAT CHAMBER DISASSEMBLY--------------------------------- 13- 9
AUTO BYSTARTER INSPECTION/REMOVAL-------------------- 13-11
AIR CUT-OFF VALVE (A.C.V.)-------------------------------------- 13-12
AUTO BYSTARTER INSTALLATION ------------------------------ 13-14
FLOAT CHAMBER ASSEMBLY ------------------------------------- 13-15
FLOAT LEVEL INSPECTION ---------------------------------------- 13-16
VACUUM CHAMBER ASSEMBLY ---------------------------------- 13-16
CARBURETOR INSTALLATION ------------------------------------ 13-17
FUEL PUMP REMOVAL/DISASSEMBLY -------------------------- 13-18
FUEL PUMP INSPECTION-------------------------------------------- 13-19
FUEL PUMP ASSEMBLY --------------------------------------------- 13-19
FUEL PUMP INSTALLATION --------------------------------------- 13-20

FUEL TANK REMOVAL ---------------------------------------------- 13-20

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13. FUEL SYSTEM/CARBURETOR/

FUEL PUMP/ FUEL TANK

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FUEL SYSTEM

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FUEL PUMP/ FUEL TANK

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SCHEMATIC DRAWING

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OPERATION OF CARBURETOR JETS

1. LOW SPEED
Venturi (slightly opened throttle valve)
Low Speed Air Inlet

※ Fuel in Float Chamber‡ Slow Jet

-------------------- Low Speed Small Jet Holes

2.MIDDLE SPEED
Venturi (halfway opened throttle valve)
High Speed Air Jet

※ Fuel in Float Chamber‡ Main Jet

Main Jet (The slow jet also works.)

Low & Middle Speed Supplementary Device:

Vacuum Tube

Low Speed Air Jet

Main Jet

Vacuum Diaphragm

Throttle

Valve

Low & Middle Speed Supplementary Fuel Inlets

Slow Jet

Bypass Jet

Pilot Screw

ACV

Air

※ Air

※ Air

Air Bleed Holes‡ Mixture…….

Air Bleed Holes‡ Mixture…….

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FUEL PUMP/ FUEL TANK

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FUEL PUMP

CONSTRUCTION:

The fuel pump adopted for this model is a vacuum-type fuel pump which utilizes the positive and
negative pulsating pressures produced by the engine crankcase to control the oil pump diaphragms
and deliver fuel from the fuel tank to the carburetor through the suction valve and outlet valve.

FUEL PUMP CONSTRUCTION

Diaphragms

Inlet

Outlet

Suction Valve

Outlet Valve

Pulsating Pressure
from Crankcase

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SERVICE INFORMATION

GENERAL INSTRUCTIONS

• When working with gasoline, keep away from sparks and flames.

• Note the locations of O-rings when disassembling and replace them with new ones during

assembly.

• Before float chamber disassembly, drain the residual gasoline from the float chamber.

• Do not try to disassemble the auto bystarter.

• When assembling the vacuum chamber and air cut-off valve, be careful not to damage the

diaphragms.

• All cables, fuel lines and wires must be routed and secured at correct locations.

• When removing the fuel tank, keep sparks and flames away from the working area.

• When removing the fuel tank, the remaining fuel in the tank must be lower than _ of the fuel tank

capacity to avoid gasoline overflowing.

• Fuel tank capacity: 10.5 liters

SPECIFICATIONS

SH50DA

Venturi dia. (mm)

CVK30

Identification number

CVK056A

Float level (mm)

18.5

Pilot screw opening

2_ ± _

Main jet

102#

Slow jet

38#

Idle speed

1700

Fuel pump output

40cc/1700rpm/10 Seconds

SPECIAL TOOLS

Float level gauge

Fuel unit remover

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TROUBLESHOOTING

Engine does not start Engine idles roughly, stalls or runs poorly

• No fuel in tank • Incorrect idle speed

• Restricted fuel line • Rich mixture

• Too much fuel getting to cylinder • Lean mixture

• Clogged air cleaner • Clogged air cleaner

• Contaminated fuel • Intake air leak

• Faulty fuel pump • Contaminated fuel

• Faulty air-cut off valve

• Damaged vacuum tube and connectors

• Damaged carburetor insulator

Throttle does not open fully, so engine stalls Rich mixture

• Damaged vacuum piston diaphragm • Auto bystarter valve opens excessively

• Clogged diaphragm hole • Faulty float valve

• Float level too high

Lean mixture • Clogged air jets

• Clogged fuel jets • Auto bystarter valve set plate installed in

• Clogged fuel tank cap breather hole

the wrong groove

• Clogged fuel filter •Clogged air cleaner

• Bent, kinked or restricted fuel line

• Faulty float valve

• Float level too low

• Faulty fuel pump or insufficient output

Engine is hard to start

• No fuel in tank

• Restricted fuel line

• Clogged fuel strainer

• Faulty fuel pump

• Broken or clogged vacuum tube

• Faulty or clogged charcoal canister

Lean mixture

• Clogged charcoal canister

• Bent, kinked or restricted fuel line

• Clogged fuel strainer

• Float level too low

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CARBURETOR REMOVAL

Remove the seat, met-in box and center
cover.
Disconnect the fuel tube and vacuum tube at
the carburetor.
Disconnect the auto bystarter wire.

Loosen the throttle cable adjusting nut and
lock nut, and disconnect the throttle cable
from the carburetor.
Loosen the air cleaner connecting tube band
and carburetor intake manifold band and then
remove the carburetor.

VACUUM CHAMBER
DISASSEMBLY

Loosen the drain screw and drain the fuel
from the float chamber.

Auto Bystarter Wire

Auto Bystarter

Fuel Tube

Intake Manifold Band

Air Cleaner Connecting Tube

Drain Screw

Throttle Cable

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Remove the two vacuum chamber cover
screws and the cover.

Remove the compression spring and vacuum
piston.

Remove the needle holder, spring and jet
needle from the piston.

VACUUM PISTON INSPECTION

Inspect the vacuum piston and jet needle for
wear or damage.
Inspect the diaphragm for deterioration and
tears.

Vacuum Chamber Cover

Diaphragm

Screws

Spring

• Be careful not to damage the vacuum

piston diaphragm.

＊

Vacuum Piston

Vacuum Piston

Needle Holder

Jet Needle

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FLOAT CHAMBER DISASSEMBLY

Remove the three float chamber screws and
the float chamber.

Remove the float pin, float and float valve.

FLOAT VALVE INSPECTION

Inspect the float valve seat contact area for
wear.

Float Chamber

Float Valve

Float

Float Pin

Float Valve Seat

Screws

Float Valve

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JETS/SCREWS REMOVAL

Remove the main jet, needle jet holder and
needle jet.
Remove the slow jet.

Clean the removed the main jet, needle jet
holder, needle jet and slow jet with detergent
oil.

Pilot Screw (P.S.)

Pilot Screw

Slow Jet

• Be sure to use clean detergent oil.

＊

Main Jet

Needle Jet Holder

Needle Jet

• Before removing the pilot screw, turn

the pilot screw clockwise until it seats
lightly and record the rotating turns.
Do not force the pilot screw against its
seat to avoid seat damage.

＊

Slow Jet

Main Jet

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AUTO BYSTARTER INSPECTION
/REMOVAL

AUTO BYSTARTER INSPECTION

Measure the resistance between the auto
bystarter wire terminals.

Resistance: 10W (10 minutes minimum
after stopping the engine)

If the reading is not within the limit, replace
the auto bystarter with a new one.

Connect a hose to the fuel enriching circuit
of the carburetor. Connect the auto
bystarter green/black wire to the positive (+)
terminal of a battery and black/white wire to
the negative (-) terminal. Wait 5 minutes and
blow the hose with mouth. If the passage is
blocked, the auto bystarter is normal.
Disconnect the auto bystarter from the
battery. Wait 30 minutes and blow the hose
with mouth.. If air can be blown into the
hose, the auto bystarter is normal.

AUTO BYSTARTER REMOVAL

Remove the one set plate screw and set plate
and then remove the auto bystarter from the
carburetor body.

Auto Bystarter

Hose

B/W

Screws

Set Plate

G/B

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AUTO BYSTARTER INSPECTION

Check the auto bystarter valve and needle
for nicks, wear or damage.
If any faulty part is found, replace the auto
bystarter with a new one.

AIR CUT-OFF VALVE (A.C.V.)

A.C.V. REMOVAL

Remove the two screws and the air cut-off
valve cover.

Remove the spring, diaphragm and O-rings.
Inspect the diaphragm and spring for wear or
damage.

Bystarter Needle

Air Cut-off Valve Cover

Bystarter Valve

Spring

Screws

Cover

Diaphragm

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CARBURETOR BODY CLEANING

Blow compressed air through all passages of
the carburetor body.

Install the O-ring onto the air-cut-off valve
body securely.

Install the diaphragm, spring, and cover.

Install and tighten the two screws attaching
the air cut-off valve cover.
Connect the hose.

Screws

Spring

Cover

Diaphragm

O-ring

• Make sure that no fuel jet is clogged.

＊

• Install the O-ring with the flat face

toward the valve body side.

＊

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ACCELERATING PUMP

DISASSEMBLY
Remove the two accelerating pump cover
screws and accelerating pump cover.
Remove the spring and accelerating pump
diaphragm.

INSPECTION
Inspect the accelerating pump diaphragm for

cracks, damage or deterioration. Replace if
necessary.

Assemble the accelerating pump in the
reverse order of disassembly.

AUTO BYSTARTER INSTALLATION

Install the auto bystarter and set plate.
Install and tighten the two screws.

Auto Bystarter

Screws

• Insert the auto bystarter into the

carburetor body until it bottoms and
position the set plate into the upper
groove in the bystarter.

• Install the set plate with its round face

facing down.

＊

Screws

Spring

Cover

Diaphragm

O-ring

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FLOAT CHAMBER ASSEMBLY

Install the main jet.
Install the slow jet.

Install the pilot screw.

Install the float valve, float and float pin.

Float Valve

Float

Float Pin

Pilot Screw

Slow Jet

Main Jet

• Be sure to record the rotating turns

when it is removed.

＊

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FLOAT LEVEL INSPECTION

Measure the float level at the location of the
main jet (just contacting the float valve).

Float Level: 18.5±1.0mm
Replace the float if the level is incorrect.

Check the operation of the float and then
reinstall the float chamber.

VACUUM CHAMBER ASSEMBLY

First install the jet needle and spring into the
vacuum chamber and then install the needle
holder.

Install the vacuum piston into the carburetor
body.

Install the spring.
Install the vacuum chamber cover and tighten
it with the two screws.

Diaphragm

Float

• Be careful not to let the diaphragm slip.

• If the diaphragm cannot be positioned

correctly because of expansion, dry the
diaphragm before installation.

＊

Spring

Vacuum Piston

Vacuum Piston

Needle Holder

Jet Needle

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Check the heater with battery.
If the heater is getting hot, means the heater

without problem, otherwise the heater has to
be changed.

CARBURETOR INSTALLATION

Tighten the drain screw.
Install the carburetor onto the intake
manifold and tighten the band.
Install the air cleaner connecting tube and
tighten the band.
Connect the throttle cable to the carburetor.

Connect the auto bystarter wire.
Connect the fuel tube and vacuum tube to
the carburetor.
Perform the following inspections and
adjustments:

•Throttle grip free play (!3-3)

•Idle speed (!3-6)

Install the seat, met-in box and frame center
cover.

Auto Bystarter Wire

Fuel Tube

Intake Manifold Band

Air Cleaner Connecting
Tube Band

Heater

• After connecting the throttle cable,

adjust the throttle grip free play to
2_ 6mm.

＊

Throttle Cable

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FUEL PUMP REMOVAL

Remove the frame center cover.
Disconnect the fuel pump inlet, outlet and
vacuum tubes.

Remove the two fuel pump attaching bolts
and the fuel pump.

FUEL PUMP DISASSEMBLY

Remove the four fuel pump body screws.

Disassemble the fuel pump.

Inlet

Screws

Fuel Strainer

Outlet Tube

Vacuum Tube

Inlet Tube

Vacuum Tube

Outlet

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FUEL PUMP INSPECTION

Inspect the fuel pump diaphragms A and B
for damage.
Inspect each gasket for damage.

Inspect the suction valve, outlet valve and
relief valve in the fuel pump body for
damage, cracks or foreign matters.

FUEL PUMP ASSEMBLY

Assemble the fuel pump in the reverse order
of disassembly.

Inlet

Spring

Suction Valve

Diaphragm
Outlet Valve

Gaskets

Relief Valve

Outlet

Dowel Pins

Dowel Pin Holes

Fuel Pump Body

• During assembly, be sure to install the

gaskets and diaphragms properly to
avoid damage.

• Do not allow any foreign matter to

enter the fuel pump during assembly.

＊

Spring

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FUEL PUMP INSTALLATION

Install the fuel pump and secure it with the
two bolts.
Connect the fuel pump inlet, outlet and
vacuum tubes.
Install the seat, met-in box and frame center
cover.

Measure the fuel pump output.
Start the engine and disconnect the fuel
outlet tube and place a clean container under
the tube to check the fuel output.

Output: 40cc/1700rpm/10 seconds .

FUEL TANK REMOVAL

Remove the floor board. (!2-4)
Remove the leg shield . (!2-5)
Disconnect the fuel unit wire connector.
Remove the fuel tube between the fuel tank
and the fuel filler.
Disconnect the fuel vapor tube.
Remove the fuel tank.

The installation sequence is the reverse of
removal.

Container

Fuel Pump

Fuel Pump

Outlet Tube

Fuel Strainer

Outlet Tube

Vacuum Tube

Inlet Tube

Fuel Unit

Fuel Vapor Tube

Fuel Strainer

Tube to Fuel Filler

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FUEL STRAINER REMOVAL

Remove the fuel strainer from the fuel tank.

INSPECTION

Inspect if the fuel strainer is clogged and
clean it with compressed air.

INSTALLATION

Install the fuel strainer with its arrow mark
toward the fuel pump.

• When removing the fuel strainer, do not

allow flames or sparks near the
working area and drain the residual
gasoline into a container.

＊

Fuel Strainer