main jet size has to do with maximum fuel delivery at wide open throttle.. not part throttle or idle.
Too much oil can foul plugs. Pull the plug and look. Get a new plug. It's suprising how often this alone cures a problem..
A small gap is prone to being bridged by drops of liquid.. fuel, oil or even water. Drops of fuel can be due to faults in the carb, most likely related to dirt somewhere..
Wide plug gapping can help a plug that's fouling.. but it's not a solution.
If, after the mixture is correct (air:fuel as well as fuel/oil) the plug continues to foul, a hotter spark plug range might be called for.
Since your engine can jump a wider gap and the bike runs better i wouldn't suspect a weak-igniton problem.