I had the 300 Twin and had to to run an on board glow to keep both cyls. burning. It was sometimes hard to notice if one side was not firing and the only way I could tell was by putting my hand close to the exhaust pipe of each cylinder and the one that was not burning was much cooler exhaust than the other. It was hard to tell even in flight until I ran the onboard glow...this may not be what is going on with your engine but I just wanted to tell you about it....other than that it was the finest engine I have ever owned in 30 years of R/C and I plan to get another 300 befroe I croke.