At idle the carburettor supplies only the mixture required to keep the engine running at very moderate rpm. The engine needs only a small amount of air when idling and the throttle slide should therefore be almost completely closed.
Upstream of the slide there is only a weak vacuum, insufficient to cause the main circuit to deliver any fuel emulsion, while downstream of the slide there is a stronger vacuum which activates the idle circuit; idle circuits are designed with either a mixture-adjusting screw or with an air adjusting screw. Check that the throttle cable has about 1 mm free play when the slide is fully closed. Always adjust the idle setting with the engine fully warm.
Screw in the idle-speed screw (4) to obtain a slightly-higher idling speed than normal (about 1200 rpm for a four-stroke engine or about 1400 rpm for a two-stroke); Then adjust the air- adjusting screw (1) to obtain the most even running.
Then unscrew the idle-speed screw again until you obtain the normal idling speed. Finally, to obtain the best engine running, it is worth rechecking by very carefully readjusting the air-adjusting screw.
3.4.1 - Idle setting with a mixture-adjusting screw
fig. 18
The adjusting screw meters the amount of mixture of a strength predetermined by the metering effect of the idle jet and the air corrector, and there fore on screwing in the mixture screw, idle fuel de livery decreases and vice-versa.
In figure 18 the throttle slide 2 is shown in the idling position, adjusted by the idle speed screw (4). In this position the vacuum present down stream of the throttle valve causes mixture to be delivered via the hole (3), regulated by the tapered tip of the mixture adjusting screw.
Mixture formed from fuel metered through the idle jet (6) and air metered by the calibrated passage (1) further mixes with air regulated by the throttle slide opening.
The idle mixture adjusting-screw is always located downstream at the throttle.
Check that the throttle cable has about 1 mm of free play with the slide closed. Always adjust the idle setting with the engine fully warmed up. Proceed as follows:
Screw in the idle speed screw (4) to get a slightly- higher speed than normal (about 1200 rpm for four-stroke engines and about 1400 rpm for two- stroke engines); then screw the mixture adjusting screw (5) in or out until you obtain the most even running. Then unscrew the throttle-stop screw (4) until you get the desired idle speed again.
To obtain the best engine running, it is worth finally rechecking by carefully readjusting the idle mixture screw (5).
3.4.2 - Idle Setting with an air-adjusting screw
fig 19
An idle circuit with an air adjusting-screw adjusts the amount of air required to produce the mixture that the idle circuit has to supply during idling.
The air adjusting screw varies the mixture strength delivered by the idle circuit; screwing in results in a richer idle mixture and vice-versa.
In figure 19 the throttle slide (2) is shown in the idle position adjusted by the idle-speed screw (4). In this position, the vacuum existing downstream of the throttle valve causes mixture to be delivered the hole (3).
Mixture formed from fuel metered through the idle jet (5) and air regulated by the idle air screw (1) further mixes with air metered by the throttle slide opening.
The idle air-adjusting screw is usually located up stream of the throttle slide.
3.4.3 - Selection of the correct size of idle jet
To select the proper size of idle jet, slowly open the throttle with the twistgrip (opening should not exceed a quarter throttle): a slow and uneven increase in rpm indicates that the idle jet is too small. This effect can also be observed when the idle mixture screw is open too much or when the idle air screw is closed too much and therefore not properly responsive to the engine's running.
If you observe smoke in the exhaust gas and a dull noise, it means that the idle jet size is too large; this can also occur when the mixture-adjusting screw is screwed in too much and oversensitive or when the air-adjusting screw is screwed out too much.
Usually with racing motorcycles, after having adjusted the idle as above, unscrew the idle- speed screw to allow the throttle to close completely so that you will obtain the maximum engine braking on closing the throttle. In this case however, do not readjust the mixture screw or air- screw setting because any further mixture screw closure or air-screw opening may cause two- stroke engines to seize on the overrun.