I have a YS 63FZ. The problem I'm having is it starts fine (cold, low compression), but when it get hot, it detonates, and then quits. I have reset the needles to factory settings, and it has a new diaphram. Pressure is good. After it quits, the compression is almost non-existant (never was what I thought is should be). I can here it going out the exhaust valve, yet the valve doesn't seem to be open at all. I did take the cyclinder off, and blew into the jug. No leaking. Here is the real mystery. After I re-assembled the engine, compression is good, and now all seems normal. I have not started it yet, but will soon. Ever heard of anything like this before? Oh yeah, it never has idle very well. I had to keep the RPM's up to keep it running.

Thanks

Bob