I have had my RCS180 for a couple of years. At the end of last year, it was running poorly. This year, I could not start it so I looked at the carb and found a lot of junk in the screen. I cleaned the screen and the engine ran like a top. It is starting easily and turning a Mejlic 20x6 prop at 8,000rpm.

Today I was flying at half throttle and the engine just quit. It turns over freely, but it lacks compression and will not fire. I took off the backplate and everything looks clean. I cannot tell if the ring is stuck, but the piston and sleeve does not look damaged. The plate, on the back of the drum, has a hole that fits over a pin sticking out of the crank pin. The hole in the plate is elongated, top to botom. Is that normal?

Can anyone suggest what I can do further evaluate this problem?