Plane is 1979 Archer II, 1250 TTAE. Just had an annual and clean plugs installed. Taxied out with mixture at about 1/2. Run up at 1800 was rough on the right mag. Ran it up to 2000 and leaned it out for about 15 seconds. Mag check now showed less than 50 RPM drop on each mag. How much should I lean after start to prevent fouling? Or do I really have another problem. Runs great in flight and seems to have no problems. Have had this problem a few time before.