There are a few things that cause plugs to foul. One thing that is most common for two strokes is oil fowling, which results from a rich mix of air and fuel. Natural wear can foul a plug simply by just getting old, when the electrode just gets worn out. And even if you have jetting perfectly, spark plugs just get build up on them. I usually go about 6 months or so on the same plug in my 250, even if it's still sparking bright blue. I would rather know the plug is fresh then foul it on the track or in the woods. And yes, a clean plug can add power, but it probably will wont be noticeable unless the old plug is ungodly fouled.
