If however it rises too much, the idle micture is too rich and if it does not rise enough its too lean.
Generally, once the engine is at normal operating temps and at idle RPM, mixture control is slowly and smoothly pulled to ICO position and the rise in RPM is noted and returned to the Full Rich position before the engine quits. If a small is rise of approx. 25 RPM is not achieved, the idle mixture screw is adjusted. Usually, in to increase RPM and Out to decrease RPM. Accelerate the engine momentarily to 1800 RPM to clear it out and recheck RPM rise at Idle Cut off. Keep in mind that adjusting the idle mixture may change the idle speed. A richer mixture will decrease the idle speed and leaner mixture will increase the idle speed.
Having said all of that, its always best to refer to and follow the instructions provided by the air frame/engine manufacturer.
Hope that helps
Thanks for the information.
From the Lycoming Overhaul Manual...
9-25. IDLE SPEED AND MIXTURE ADJUSTMENT-
(Except Simmonds FuelInjector). See figures 9-1 and
9-2. With engine thoroughly warmed up, check magneto
drop- off. If the drop- off is excessive, check for fouled
plugs. If drop- off is normal, proceed with idle adjustment.
Close the throttle, engine should idle at approximately
600 RPM (turbocharged engines, approximately
1000 RPM). If the RPM increases appreciably
after a change in the idle mixture adjustment during
the succeeding steps, readjust the idle speed adjustment
to restore the desired RPM.
9-26. Move the mixture control lever with a smooth,
steady pu II into the idle- cut- off position and observe the
tachometer for any change during the leaning process.
Caution must be exercised to return the mixture control
lever to "full rich" before the RPM can drop to a
point where the engine cuts out. An increase of more
than 50 RPM while "leaning out" indicates an excessively
rich mixture. An immediate decrease in RPM
(if not preceded by a momentary increase) indicates
the mixture is too lean.
9-27. If it is indicated that the mixture is either too
lean or too rich, turn the idle mixture adjustment one
or two notches in the direction required for correction.
Check this setting by repeating the above procedure.
Each time the adjustment is changed, the engine should
be run up to 2000 RPM to clear the engine before proceeding
with the idle speed check. The final adjustment
of the idle speed should be made with throttle
10-5, MARVEL-SCHEBLER CARBURETOR
If after checking all other possibilities, it is found
necessary to readjust this type of carburetor,
proceed as follows:
a. Start and warm up the engine in the normal
b. Set throttle stop screw to obtain an idle speed
of 500 R.P.M.
c. Turn idle adjusting needle out slowly until
engine "rolls" from richness, then turn in
slowly until engine lags, or runs irrugularly
from leanness. This step will determine the
adjustment range and how the engine operates
under extreme idle mixtures.
d. From the lean setting, turn needle out slowly
to the richest mixture that will not cause the
engine to "roll" or run unevenly. This
adjustment will in most cases give a slower
idle speed than a slightly leaner adjustment
with the same throttle stop screw setting, but
will give smoothest idle operation. A change
in idle mixture will change the idle speed and
it may be necessary to readjust the idle speed
with the throttle stop screw to the desired
point. The idle adjusting needle should be
from 3/4 to 1 turn from its seat to give
satisfactory idle mixture.
Care should be taken not to damage the
idle needle seat by turning the idle
adjusting needle too tightly against seat, as
damage to the seat will make a satisfactory
idle adjustment very difficult.
Hope some of that helps. If you need anything else, or if you need something for a particular model engine let me know or send me a PM and Ill do my best to dig something up for you.