With out getting to much in timing, if it was retarded it shouldnt keep running on when shut off, it would be missing and sometimes a backfire, advance unit may be sticking.
It sounds like a jetting problem. Go down 3 jet sizes if really black on the plugs. Just because the carb ran good on one engine dont mean it will run great on another even if the same type.
Holly has a free downloadable manual and troubleshooters guide that you may want to print out. Please email me if you need any other help.