It could be the oring on the coolant pipe that goes into the side cover,could also be that you have you case vent hose hooked up wrong.You could have it hooked together with the water pump vent and the over flow hose.
It could still be the stator,you have an ignition windings and lighting windings on the stator.It also could be your voltage regulator,or wiring, the switch.Do you have voltage to the yellow or green wires?Check your grounds also.
NO it is NOT.If you have more oil in your cylinder then you have less room for fuel therefore it means you are leaner on your fuel mixture.More oil is lean,less oil is richer!
The more oil you have the leaner you are,the less oil you have the richer you are.If you have more oil mixed with your fuel then there will be less fuel in your cylinder causing it to be leaner.
I think the 18cc will give you to much compression.A 21cc dome at sea level would give you about 180 with that set up.I would not go smaller than a 20cc.