From your description, it sounds like the idle mixture is too rich. Possible causes of that condition are the idle mixture screw is incorrectly set, or there is internal leakage that affects the idle mixture. Try setting the idle mixture screw a half turn in (lean). If you set the mixture screw for the highest idle vacuum, it is easy to turn the screw out (rich) a half turn or so without affecting the manifold vacuum. Also, if you set the mixture before the engine reached normal operating temperature, you could have the mixture set too rich for the hot idle.
Harvester
