The butterfly flap in the idle control/throttle valve can become contaminated with gunk from the cambox breather on these engines and can cause rough idling problems. Disconnect the cross tube over the cambox for access and clean out the throat and the butterfly valve with meth spirit or similar solvent on a cloth and/or brush.
Also check the throttle return springs are in place and not broken,there is a coil spring on the front of the throttle valve which breaks and drops down, not obvious unless you know it's there.
Geo.