How to tune up
|Sheet 1||Illustrations of Carburettor types 375,376,389 & 689||How it works & part names||General hints & tips|
|Re-assemling & How to trace faults||Parts to tune up with||How to tune up||Tuning twin engines with twin Carburettor|
SEQUENCE OF TUNING
TUNE UP IN THE FOLLOWING ORDER ONLY, by so doing you will not upset good results obtained.
MAIN JET with throttle in position 1 (sheet
If at full throttle the engine runs 'heavily', the main jet is too large. If at full throttle by slightly closing the throttle or air valve, the engine seems to have better power, the main jet is too small. With a correct sized main jet, the engine at full throttle should run evenly and regularly with maximum power. If you are testing for speed work, ensure that the main jet size is sufficient for the mixture to be rich enough to keep the engine cool, and to verify this, examine the sparking plug after taking a fast run, de-clutching and stopping the engine quickly. If the plug body at its end has a cool appearance, the mixture is correct, if sooty the mixture is rich, if however there are signs of intense heat, the mixture is too weak, and a larger main jet is required.
PILOT JET WITH THROTTLE IN POSITIONS 2 AND 5.
With the engine idling too fast with the twist grip shut off and the throttle shut down on to the throttle adjusting screw, and ignition set for best slow running; a) screw out the throttle adjusting screw until the engine runs slower and begins to falter. Then screw the pilot air screw in or out to make the engine run regularly and faster. b) Now gently lower the throttle stop screw until the engine runs slower and just begins to falter, and begin again to adjust the pilot air screw to get the best slow running. If this second adjustment makes the engine run too fast, go over the job again a third time.
3rd. THROTTLE CUT AWAY with throttle in position 3 (sheet 7 ). If, as you take off from the idling position, there is objectionable spitting from the carburetter, slightly richen the mixture by screwing in the air screw sufficiently, but if this is not effective, screw it back again, and fit a throttle with a smaller cut away. If the engine jerks under load at this throttle position and there is no spitting, either the throttle needle is too high or a larger throttle cut away is required to cure the richness.
4th. NEEDLE with throttle in position 4 (sheet 7 ). The needle controls a wide range of throttle opening and also the acceleration. Try the needle in as low a position as possible, eg: with the clip in a groove as near the top as possible. If acceleration is poor and with the air valve partially closed the results are better, raise the needle by two grooves. If much better, try lowering the needle by one groove and leave it where it is best. NOTE, if the mixture is still too rich with the clip in groove number one (nearest the top), the needle and needle jet probably needs replacing due to wear.5th FINALLY go over the idling again for final touches.
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