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
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.