Assuming everything checks out OK and you can't isolate the problem, I'd suspect engine tuning problems. First, have you run the engine on the ground to break it in? I believe NextStar says the engine is broken in, but I wouldn't believe their claim. I'd run a couple of tanks of fuel through it to ensure its reliable operation in the air. Next make sure the engine is set on the rich side. Max engines won't run reliably if they are lean. After every flight move the needle valve in a click until you reach what Max calls the optimum RPM. This is 15-30 degrees away from the maximum RPM (Where the engine is leaned out for max RPM).

Incidentally, I own three OS engines: 40LA, 46 AX, and an old 19. All of them run great, and the 40 and 19 have had many gallons run through them.

Ciao,

Eagle Al