O.S. Engines 7D Carb - 29081000 - Manual

INSTRUCTIONS FOR THE O.S. TYPE 7D CARBURETTOR
This carburettor incorporates an automatic mixture control device which ensures that the engine receives a
correctly balanced mixture of fuel and air at all throttle settings. It provides steady revolutions and a
smooth response, even to abrupt operation of the throttle. Adjustment, when required, is very easy to carry
out.
1.5~2.0mm
Fig.(1)
Three adjustable controls are provided on the Type 7D
carburettor. (See photo.)
• The Needle-Valve: For adjusting the mixture strength
when the throttle is fully open.
• The 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 Throttle Stop Screw: For setting the position where
the carburettor rotor is closed.
PROVISIONAL SETTING
The use of a muffler-pressurized fuel system is recom-
mended. Open the Needle-Valve 1 1/2~2 turns from the
fully closed position before starting the engine.
ADJUSTING THE CARBURETTOR
1) Set the throttle very slightly open from the idle position
(See Fig. 1) and start the engine in the usual way. It is
preferable to have the throttle only slightly open, to avoid
unnecessarily high revolutions when the engine starts.
2) Now open the throttle fully and gradually close the
Needle-Valve until the engine is running at its maximum
speed.*
*Warnings
(a) Do not close the Needle-Valve to too "lean" a setting, as this
will cause overheating which may result in internal damage
and reduced engine life. Set the Needle-Valve approximately
1/4 to 1/2 turn open (i.e. "rich") from the peak r.p.m. setting,
even though this will slightly reduce r.p.m. on the ground.
(b) Make sure that the engine is fully "run-in" before operating it
continuously at full power. (See engine instruction leaflet.)
3) Having set the Needle-Valve as described above, close the
throttle. The engine should idle steadily and continuous-
ly. (If it stops immediately, first try turning the Throttle
Stop Screw clockwise a few degrees to raise the idling
speed a little.)
(a) If, however, the engine idles unevenly and smokes a
good deal when the throttle is re-opened, before
picking up speed, it is probable that the idling mix-
ture is too rich. In this case it will be necessary to
adjust the Mixture Control Valve by turning the
Mixture Control Screw 1 5 to 30 degrees clockwise.
(b) Alternatively, if the Mixture Control Valve is set too
lean, the engine may stop when the throttle is
closed, or it may idle for a few moments, then speed
up very slightly before coming to a stop. Re-opening
the throttle under these conditions will usually
result in the engine cutting out abruptly, without
smoking. Corrective action is to turn the Mixture
Control Screw about 1 5 to 30 degrees counter-clock-
wise.
Mixture Control Valve adjustment is not unduly critical,
but do not turn the screw more than 15 — 30 degrees at
a time.
Carry out adjustments progressively and patiently, re-
membering the symptoms of rich and lean running,
quoted above, until the engine responds quickly and
positively to the throttle. Do not alter the Needle Valve
setting while adjusting the Mixture Control Valve.
Warning:
For safety reasons, it is advisable to stop the engine before
carrying out adjustments to the Mixture Control Valve or
Throttle Stop Screw.
Throttle Stop Screw
Mixture Control Screw
Needle Valve
Photo (1)