Setting the Idle Mixture & Idle Speed
The engine should be at optimum operating temperatures. Lightly seat the idle mixture screws, then back them out 1-1/2turns to establish a starting point. With the engine running, slowly turn the mixture screws in or out as needed to establishthe best idle quality. Do this twice. The first time is a coarse adjustment; the second one is a final fine adjustment.
If idle quality can't be adjusted properly, or if the idle mixture screws can't be backed out far enough to obtain properadjustment, it may be necessary to have the idle circuits reworked (the cam may not be pulling enough idle vacuum).
A common reason for lack of adjustability in the idle mixture screws is having the primary butterflies adjusted too far open,which prevents the idle mixture screws from being capable of controlling the idle quality. When setting idle speed, thebutterfly openings should be adjusted equally both primary and secondary side. However, the transfer slot should not have more 0.040" exposed (viewed with the carburetor removed from the engine and turned upside down). Secondary adjustment is set with a screw accessible from the underside of the carburetor.
Remember, the secondary throttles should be just slightly cracked open at idle. Even on carburetors with non-adjustablesecondary idle mixture, secondary throttle position at idle may be adjusted. It may be necessary to open or close the throttleadjustment due to the cam design. It's a trial-and-error proposition to find the setting that gives you the best idle. However, here is a starting guideline. On engines that idle at or above 1000 rpm, start with the primary and secondary butterflies open the same amount. For engines that idle below 1000 rpm, begin with the primary butterflies open about 0.020" and the secondary butterflies positioned at the bottom of the transfer slot. From these points, begin to adjust the idle adjustment screws until the idle is steady and smooth.


Back to the toolbox!

Back to!