I am somewhat of a novice (been flying for about a year) and I am having a "no compression" problem with my ST g51 similar to another poster. I have a ST 90 that I broke in according to the instructions and am very happy with it. I recently purchased a ST g51 and broke it in according to the instructions - open needle valve 3 turns, run very rich (fuel was spitting out of the carb) for 5 minutes. Lean out 1/8 turn, run for 1 minute, do pinch test, lean out 1/8 turn, run 1 minute, do pinch test... and so on.

During the fourth tank it broke into 2 cycle running and was running strong. During the fifth tank I checked out the idle and mid-range, which seemed good. However, it started losing power at full throttle and when the engine stopped and cooled I discovered that I had hardly any compression. My questions are:

1. Could I have done something wrong? What?
2. Is it possible that the engine was defective? Should I ask ST for a new one?
3. Can I fix it by replacing the piston ring, or is it possible the cylinder sleeve is messed up? Is it hard to change a piston ring? Do you need special tools?

Thanks.