Are you loosing any trans oil? Could be a right side seal causing the engine to suck trans oil and foul the plug. Even a new seal can get nicked upon installation of the crank!
Does this thing foul the plug like immediately? If so, is it really black and wet? I have had a bad stator cause this before. I could put in a new plug and in like two minutes it would be fouled (black and wet). After about four plugs in 20 minutes one day, it would no longer fire at all, even with a new plug. Turned out to be the pulse coil on the stator was going bad and had finally gone open, thus I no longer had any fire at all. I know this is not the normal, but just an experience that I had on a two stroke Honda.