What is causing my Briggs & Stratton lawn mower engine to misfire?

My Briggs & Stratton lawn mower was manufactured in 1994. Not to sure of engine style, but 0625-01 is the type #. Model # is: 12E702. The carb is the basic Briggs & Stratton float type carb. The mower is second hand. No manual.

Orginally, after replacing the spark plug and getting the mower started, there was an immediate problem with the engine surging at full power and misfiring. The following tasks were carried out to resolve the problems:

1. Cleaned the carb fuel bowl and securing jet nut.
2. Adjusted carb fuel mixture jet to 1 1/2 turns open instead of 1 turn open.
3. Magento was badly scarred for some unknown reason, so have given it a light clean with a bit of P800 emry paper.

Engine no longer is surging and idles pretty well; however, it is still misfiring.

Does anyone have any ideas how to resolve this problem?.. Could the scarred magneto be causing the misfire? If so, would more cleaning with the emry help? All the electrical wires seem to be well secured.

Mike
August 2008