TUNE UP IN THE FOLLOWING ORDER.
NOTE. The carburetter is automatic throughout the throttle range - the air valve should always be wide open except when used for starting or until the engine has warmed up. We assume normal
petrols are used.
READ REMARKS ON PAGES 6 AND 7 for each tuning device and get the motor going perfectly on a quiet road with a slight up gradient so that on test the engine is pulling.
1st. MAIN JET with throttle in position 1 (fig. 5).
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 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, declutching 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 necessary.
2nd. PILOT JET (fig. 5) with throttle in positions 2 and 5.
With 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: (I) Screw out throttle adjusting screw until the engine runs slower and begins to falter, then screw pilot air adjusting screw in or out. to make engine run regularly and faster. (2) Now gently lower the throttle adjusting screw until the engine runs slower and just begins to falter, adjust the pilot air adjusting screw to get best slow running : if this 2nd adjustment make engine run too fast, go over the job again a third time. Both the throttle adjusting screw and pilot air screw have an "O" Ring fitted to hold the adjustment by friction.
3rd. THROTTLE CUT-AWAY with throttle in position 3 (fig. 5) If, as you take off from the idling position, there is objectionable spitting from the carburetter, slightly richen the pilot 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 jet needle is much too high or a larger throttle
cut-away is required to cure richness.
4th. NEEDLE with throttle in position 4 (fig. 5).
The needle controls a wide range of throttle opening and also the acceleration. Try the needle in the lower position, viz., with the clip in the groove at the top; if acceleration is poor and with air valve partially closed the results are better, raise the needle by two grooves; if very much better try lowering needle by one groove and leave it where it is best. If mixture is still too rich with clip in groove No.
1 nearest the top - the needle jet probably wants replacement because of wear. If the needle itself has had several years' use replace it also.
5th. FINALLY go over the idling again for final touches.