Sorry Bax, I've been away and just got to read your reply. Yes, I agree the knocking noise could be fuel burning in the muffler. As far as the fuel system is concerned that's NOT the problem. I've done enough testing to eliminate that as a possible cause. I will try some different plugs in it and I suspect that may be the problem since it doesn't tolerate being rich at all. This is the most sensitive engine I've owned when it comes to being a little rich. At the point of just starting to four stroke it will not staying running at all in the air. Guaranteed to die. The most reliable flights are when it is on the verge of being too lean.

I was hoping for an easy answer but I will pick away at it until it's figured out. Thanks again Bill and when I get this solved I'll post it back. As I mentioned above, I strongly suspect the plug is too cold because it doesn't tolerate being rich at all and I mean absolutely NO four stroking That usually spells too cold of a plug in my past experience or possibly too large of fuel droplets requiring an idle bar to keep them from extinguishing the glow plug element.

Best Regards,
Butch