Maybe the idle mixture is set way too rich? If the gearlube is full, clean and no water, it must be unburned fuel mix. I don't use muff's often, but that is quite a bit of build-up. Maybe normal if you ran on muffs for quite awhile.. I don't know.
If your motor is pre-'77, it has points ignition. Perhaps the points are set wrong. Should be .020" at widest gap on cam. If it's less, it'll be timed 'late' and perhaps not burn completely.. or maybe gapped too wide and pre-igniting? Just throwing it out there
Do your final tuning in a barrel or on the lake where you have exhaust back-pressure..