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3.4. Idle systems
At idle the Carburetor 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
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.
3.4.2 - Idle Setting with an air-adjusting screw
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.
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