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.

It's really hard to say what happened. If the engine was running rich of peak RPM all the time, it should not have overheated, which is what usually causes loss of compression in a ringed engine.

It's possible the engine went overlean from some other factor, and that would cause the overheat. It's also possible that something caused the ring to no longer seal. The engine could have ingested something that scored the piston/ring/liner assembly to the point it won't hold compression, or the ring has broken for some reason. Only a teardown will tell.

No special tools are needed to disassemble and reassemble the engine, but you do have to make sure the ring gap is over the pin in the ring groove of the piston, and that the piston is aligned when you install the liner.

If it's a possible warranty concern, then the engine should come into the service center.