ORIGINAL: AutoMike
I know you've just about pulled your hair out on this engine, but I have an idea. In two strokes, I've only run K&B and Fox engines for years now and experimenting with fuel, props and plugs, I've found that the American engines just perform most reliably, overall, on idle bar plugs. Fireball, K&B and Fox idle bar plugs all work well with Fireball and K&B appearing to be ever so slightly better in SOME engines. Don't want to start a plug debate here, but you didn't seem to give any info on the plug your using, but give one of these a try and note any change, hopefully for the good.
Idle bar plugs are designed for non schneurle engines with baffles on the piston top. The baffle directed the incoming fuel from the case to the top of the cylinder. That however, could cause the glow plug to be cooled by that charge. The idle bar helped shield the glow element form that incoming fuel charge. Schneurle engines angle the inlet ports to swirl the mixture and have a flat top piston, so glow plug cooling should not be an issue.
That said, I have used idle bar plugs in non baffle engines and have not had any problems. I just don't know if it would solve any problems, since the.65 is a schneurle design, is it not? It won't hurt to try though. Another option is a Fox Miracle plug also known as a hose nose, which sets the glow element in a kinda recess. I think the OS F plug is like this !?!.
Good luck, I have K&B engines .29, .35, .40, .61 and .65 and love em all.
Regards, Richard