Originally Posted by afm

Some racers make the mistake of setting the idle speed opening to wide (high idle) and set the bottom end too rich. This will give a false normal idle speed, even though the speed is set to high, because it “loads up” the engine with excess fuel causing the idle to be lower than set. The end result is a very unstable idling engine that surges and may cut out as full throttle is applied because the over rich bottom end can disguise a too lean top end setting!

It is possible too set the idle screw adjustment in to far, but yet the idle speed is not high!, even though the air regulation (carb barrel or slide) may be set to a position that would normally equal a vary fast idle, the idle is low because the low speed mixture adjustment is set so rich that the engine loads up with excess fuel, and the engine goes into what’s commonly called a four cycle idle.

One tell-tale sign of this is if after revving up the completely warmed up engine, it tends to idle fast for a few seconds, then drops to a lower idle speed. In other words, it’s something like da.da.da.da.da.da.da.da then it drops to da….da….da….da.
So you must start leaning the bottom end a little at a time (then repeat the revving up and idle test), and if it takes longer before the idle drops, you’re going in the right direction. Eventually as you keep leaning the bottom, the idle will stay high, and not drop. Now it is time to lower the idle to where it belongs by re-adjusting the idle screw.

Now that you have the idle set correctly, the top end may be too lean! so readjust accordingly

AFM