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Jetex Carburettor
ADJUSTMENT OF THE CARBURETTOR
 
Check and ensure that the carburettor is fitted with the float chamber in a perfectly vertical position and that both inlet manifold and carburettor are well secured to avoid air leaks.
Adjustment of slow running :
This must be done with a warmed up engine. The adjustment of the slow running is carried out through 2 screws, one adjusts the position of the throttle, (fig.47 screw 11) and the other controls the quantity of the mixture which is formed at the exit of the slow running jet. (By turning the screw clockwise the mixture is lean and by turning it anti-clockwise the mixture is rich).
Obtain the correct adjustment of the slow running mixture as follows:
1 Adjust the screw 11 which controls the position of the throttle in such a way as to obtain a slightly fast idling.
2 Adjust screw 35 so as to obtain the most regular running of the engine. If the engine races this indicates a rich mixture. If the screw is tightened the engine will have a tendency to stop, this indicates weak mixture, therefore loosen screw 35
3 Adjust therefore, screw 11 and 35 alternatively until the best slow running condition is obtained.
 
Usually screw 35 should be unscrewed between 1/2 to 3/4 turns from the point at which it is fully closed. Once this slow running is adjusted and by opening the throttle the engine tends to miss or stop, this indicates a weak mixture and screw 35 must therefore be loosened very slightly until this inconvenience is eliminated.
 
All contents in this section is taken from, Lambretta GP150/GP200, Workshop Manual, Instructions for repair shops, Scooters India Limited, Lucknow (India).