O.K.
So when it DOES start, what do you do to get it to start?
Just narrowing down the problem a little bit.
Right now I'm leaning on a fuel delivery problem, as the other two (spark and air) aren't likely to be variable or intermittent aside for a CDI, coil, or short in the loom.
When it runs, does it run well? Does it backfire? How are you mixing the fuel? What kind of fuel? What kind of oil/gas mix? Does it backfire? Does it idle without stalling? For how long?
etc., etc., etc.