O.S. Engines 21VZ-B - Manual (Page 9)

16
Warning!
Mixture adjustments (whether via the Metering
Needle, 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.
With the optimum mixture control position, light
smoke is visible during high speed running,and the
engine revolutions increase smoothly during
acceleration. Remember that, if the engine is
operated with the fuel/air mixture slightly too lean, it
will overheat and run unevenly. As with all engines,
it is advisable to set both the needle-valve and
metering needle very slightly on the rich side of the
best rpm setting, as a safety measure.
If the engine runs too fast with the throttle closed,
the throttle stop screw should be turned counter-
clockwise to allow the throttle opening to be
reduced.
Finally, beyond the nominal break-in period, a slight
readjustment toward a leaner needle setting may be
required to maintain performance.
With the engine running, close the throttle and
allow it to idle for about five seconds, then reopen
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 Metering Needle clockwise
45-90˚. 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 Metering
Needle counter-clockwise 45-90˚.
NOTE:
Metering Needle adjustment should be made in
steps of not more than 45-90˚, carefully checking
the effect,on throttle response, of each small
adjustment.
Carry out adjustments patiently, under actual
running conditions, until the engine responds
quickly and positively to the throttle control.
17
Mixture Control Valve
Carburetor Body
CARBURETOR CLEANLINESS
The correct functioning of the carburetor depends on
its small fuel orifices remaining clear.
(See Care and Maintenance section on Page19.)
Slide Valve
Carburetor Body
Metering Needle
REALIGNMENT OF METERING NEEDLE AND
MIXTURE CONTROL VALVE
First rotate the Metering
Needle until its slotted head is
flush with the carburetor body.
This is the standard position.
In the course of making carburetor adjustments, it is
just possible that the Metering Needle and 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 rotating the
Metering Needle and Mixture Control Valve until its
slotted head is flush with ball link the carburetor body.
Ball Link
First rotate the Mixture Control
Valve until its slotted head is
flush with the carburetor body.
Then screw the valve in exactly
1 turn.
This is the standard position.