There seems to be a ghost or a gremlin in my engine. I have an older K&B .61 that I got second hand a while back. It's a strong engine when it wants to be. I've noticed something though. Sometimes it will have awesome compression, then other times it doesn't seem to have any. I had to replace the bearings a couple months ago, and I noticed that the sleeve is in almost perfect condition. I removed the piston and cleaned out the slot the ring sets in. Everyting internally looks fine. I had it out this afternoon and I noticed that it had good compression through the first 3 flights. When I went to start it for the 4th flight, it had very weak compression. It started and ran, but kind of sluggish. About 1/2 way through the flight, it just took off and started revving up like it's supposed to. When I landed, it had good compression again. I would think if it had a cracked crankcase, gouged up sleeve or a weak ring that it would have poor compression all the time. I haven't taken the head off since the rebuild, but I wouldn't think the sleeve would be turning. Anyone else have any ideas?