O.S. Engines 20B User Manual

INSTRUCTIONS FOR O.S. TYPE 20A & 20B AUTOMATIC CARBURETTORS
These slide-throttle carburettors have been designed expressly for O.S. racing car engines. They release the full
potential
output.
The
shank of
mission of heat from the crankcase to the carburettor body. This prevents excessive pre-heating of the incom-
ing
charge
and maximum power output.
of
these engines,
the
carburettor
is
and maintains steady
providing
fitted
with
running
instant
an
insulating
characteristics
response,
sleeve
with
rapid
rapid
acceleration
of
engineering plastic
deceleration,
as
well
and
to
minimize
as
instant
high
peak
power
the
trans-
acceleration
When supplied with the engine, the carburettor is fitted loosely in the intake boss. Secure it in the following manner.
1. Loosen the carburettor retainer screw and rotate the carburettor to its proper position in the intake boss.
2. The carburettor is aligned vertically and sealed against air leaks by a sealing washer in the bottom of the intake boss and by a heatproof rubber gasket between the carburettor body and the rim of the intake boss. Therefore, press the carburettor firmly down into the intake boss while rotating the retainer screw. Rotate the screw gently until it stops, then tighten a further 120~180°.
Note: The two-piece retainer is designed to pinch the
carburettor from both sides and the insulating sleeve effectively prevents the assembly from vibrating loose.
Warning! Do not over-tighten the retainer screw as
this will damage the insulator.
Important: These carburettors require muffler pres-
surized fuel feed. Make sure that the fuel tank is completely sealed when filled and that there are no leaks in the pressure line or delivery pipe.
Needle Valve
Mixture Control Valve (Mixture Control Screw)
Throttle Stop Screw
Photo1
ADJUSTING THE CARBURETTOR
Three adjustable controls are provided on this carbuettor.
• The Needle Valve:
When set to produce maximum power at full throttle, this establishes the basic fuel/air mixture strength, which is then maintained by the carbu­rettor's automatic mixture control system to cover the engine's requirements at reduced throttle set­tings.
• The Mixture Control Valve (Mixture Control Screw):
For adjusting the mixture strength at part-throttle and idling speeds, to obtain steady idling and smooth acceleration to medium speeds. The Mix­ture Control Valve has been factory set for the approximate best result. First, run the engine as received, and re-adjust the Mixture Control Valve only when necessary.
• The Throttle Stop Screw:
For setting the position where the carburettor rotor is closed.
0.3-0.5mm
Fig.1
ADJUSTMENT
Note: Optimum carburettor performance is possible
only after the engine has been adequately run­in - as described in the engine instruction leaflet. Carburettor adjustment procedures should then be carried
out
(with
the
air
cleaner
fitted,
where appropriate) as follows.
1. Use the same fuel as you intend to use for normal running.
2. Open the Needle-Valve 2 1/2 turns (20A carburettor) or 3 turns (20B carburettor) from the fully closed
position. Set the throttle very slightly open from
the idling position (0.3mm~0.5mm from the fully closed position). Start the engine and allow it to warm up.
3. Now check that the Mixture Control Valve is set slightly rich, i.e. so that, after idling for five seconds, the engine hesitates, when the throttle is opened, before picking up speed. If the engine stops due to being over-rich, set the idling speed slightly higher by means of the Throttle Stop Screw. Operate the car smoothly, avoiding abrupt throttle movements at this stage. If, however, the engine still stops through being over-rich, close the Needle-Valve about 30° and try again.
4. Run the vehicle with this needle-valve setting (and with throttle fully open) over the longest available straight course, in order to observe the model's speed. Next, return the car to the starting point, close the Needle-Valve 30° and repeat the run, taking note of the improvement in performance. Continue with further runs, gradually reducing the
Needle-Valve setting and aiming to achieve the highest straight-line speed. Remember, however, that, if the Needle-Valve is shut down too far, the engine will overheat and, accompanied by visibly diminished exhaust smoke, the model will lose speed. At this point, throttle down immediately, stop the vehicle and reopen the Needle-Valve 30~ 45°.
5. Re-check performance, making small readjustments to the Needle-Valve, until a setting is found that gives the highest speed without overheating, then open the Needle-Valve 20~30° as a safety margin. This is the optimum setting, although it may need to be altered if a different fuel is used.
6. Having established the optimum needle-valve set­ting, check the Mixture Control Valve setting as follows.
7. With the engine running, close the throttle and allow it to idle for about five seconds, then open the throttle fully. If, at this point, the engine puffs
out an excessive amount of smoke and the vehicle does not accelerate smoothly and rapidly, it is probable that the idling mixture is too rich. In this case, turn the Mixture Control Valve clockwise 30°. If, on the other hand, the engine tends to speed up momentarily and then cut out abruptly when the throttle is opened, the idling mixture is too lean. Correct this by turning the Mixture Control Valve counter-clockwise 30~45°.
Note: Mixture Control Valve adjustment should be
made in steps of not more than 30°, carefully checking the effect, on throttle response, of each small adjustment.
8. Carry out adjustments patiently, under actual run­ning conditions, until the engine responds quickly and positively to the throttle control.
9. With the optimum settings, light smoke is visible during high-speed running and engine revolutions increase smoothly during acceleration. Remember that, if the engine is operated with the fuel/air mixture only slightly too lean, it will overheat and run unevenly. As with all engines, it is advisable, as previously noted, to set both valves very slightly
on the rich side of the highest r.p.m. setting, as a safety measure.
10. If the engine runs too fast with the throttle closed, the Throttle Stop Screw should be turned counter­clockwise a few degrees to allow the throttle open­ing to be reduced. Readjustment may be necessary to suit alterations in gear ratios or the clutch engagement point.
SUBSEQUENT READJUSTMENTS
Mixture adjustments (whether via the Mixture Control Valve, or the Needle-Valve) cannot be made accurate­ly under 'no load' conditions, which, in any case, are not advised, since such operation carries the risk of seriously damaging the engine through over-revving and overheating.
Once the engine has been run-in (see engine instruc­tions) and the carburettor controls properly set up, it should be unnecessary to alter the mixture settings, except to make minor adjustments to the Needle­Valve occasionally, to take account of variations in
climatic conditions.
The use of a different fuel, however, particularly one containing more, or less, nitromethane and/or a different type or proportion of lubricating oil, is
likely to call for some readjustment of the Needle­Valve.
Remember that, as a safety measure, it is advisable to increase the Needle-Valve opening by an extra
half-turn counter-clockwise, prior to establishing a new setting. The same applies if the silencer type
is changed. A different silencer may alter the exhaust pressure applied to the fuel feed and call for a revised
Needle-Valve setting.
'Flats' for 8mm wrench
Slide Valve
•Flats' for 6mm wrench
Slide Valve Extension
CARBURETTOR CLEANLINESS
The correct functioning of the carburettor depends
on its small fuel orifices remaining clear. The minute particles of foreign matter that are present in any fuel, can easily partially obstruct these orifices and upset mixture strength so that engine performance becomes erratic and unreliable. O.S. 'Super-Filters' (large and small) are available, as optional extras, to deal with this problem. One of these filters, fitted to the outlet tube inside your refueling container, will prevent the entry of foreign material into the fuel tank. It is also recommended
trial a good in-line filter be installed between the tank
and carburettor. Do not forget to clean the filters regularly to remove dirt and lint that accumulate on the filter screens. Also, clean the carburettor itself occasionally.
Fig.2
First rotate the Mixture Control Valve until its slotted head is Hush with the carburettor body
1.
0.5mm(20B)
Then screw the valve in exactly 1 turn (20B carburettor), or 3 turns (20A carburettor).
Mixture Control Valve
Carburettor Body
mm
(20
BALL LINK AND SLIDE VALVE EXTENSION
When readjusting the position of the Ball Link, always apply a 6mm spanner or wrench to the flats on the Slide Valve Extension, before attempting to loosen or tighten the ball link retaining screw. Care­fully ease back the Dust Cover bellows to allow access to the flats. If it should become necessary (e.g. for cleaning) to disassemble the throttle parts, first
2 turns
A)
1 turn
unscrew the Slide Valve guide screw and withdraw the complete sub-assembly from the carburettor body. Use the correct size (8mm) wrench when unscrewing the Slide Valve from the Slide Valve Extension. See Fig. 2 and parts drawing (below).
Fig.3
REALIGNMENT OF MIXTURE CONTROL VALVE
In the course of making carburettor adjustments, it is just possible that the Mixture Control Valve may be inadvertently screwed in or out too far and there­by moved beyond its effective adjustment range.
Its basic position can be found by first rotating the Mixture Control Valve until its .slotted head is flush with the carburettor body. The valve is then screwed in exactly 1 turn (20B carburettor), or 3 turns (20A carburettor), to establish its neutral position. See Fig. 3 (above, left).
PARTS LIST
Description
Throttle Stop Screw (w/spring) Mixture Control Valve Assembly "0"Ring(L) for Mixture Control Valve "0" Ring(S) for Mixture Control Valve Needle Valve Assembly
"0" Ring for needle Universal Nipple No.9(w/sealing washer) Carburettor Body (fitted with Thermo-lnsulator) Slide Valve Guide Screw Carburettor Rubber Gasket Carburettor Sealing Washer Slide valve
Metering Needle Assembly (w/spring) Slide Valve Extension Dust Cover Ball Link No.1
The specifications are subject to alteration for improvement without notice.
6-15 3-chome Imagawa Higashisurniyoshi-ku Osaka 546, Japan. TEL (06) 702-0225
© Copyright 1995 by O.S. Engines Mfg. Co., Ltd. All rights reserved. Printed in Japan
Code
No.
20A
23818500 23818180 46066319 22781800 23818150 46066319 23818176
23818100
45581820 29015019
23818190 23818201 23818300
23818211
22884210
23818400
FAX (06)704-2722
20B
23818110
23818251 23818310
60130180 29507
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