What would cause a 30 size engine to lose compression in the middle of running. I am pretty sure that it was not too lean. I am running 15% cool power fuel in it, and it was running good enough to fly 2 days ago. Thanks for any help you could give.

Check your clunk line it may have colapsed.

I wouldn't think that a colasped clunk line would cause the engine compression to go away? There is still no compression even after it was off. Engine will not start.

check your head bolts but it sounds like lean running has killed this one

I had a heli that the clunk line colapsed and the engine leaned out enough to sieze the motor before I could get it on the ground. It messed the sleeve up and thus the loss of compression.

I would hate for you to get the engine fixed and have the clunk line ruin another.

If it is a ringed version some have lower compression on start-up to counter starting issues like seen on some ABC larger engine which can be rough on your starter shafts. But it sounds like you need to check-re-torque all engine bolts especially head bolts and go from there. If it really seems to have a loss of compression you might need to tear-down and check the ring in F series and if ringed version of newer OS, or if ABC which most are on newer engines look at the piston & sleeve to look for damage or wear and check to see if you need a piston or liner or ring..according to if it's ringed or ABC type engine..

This is after you exhaust all other issues addressed here first..

Upon taking this engine apart to see what was wrong, I found that the ring had broken. Killed the piston, ring and rod. To replace these parts will cost about $70, so it looks like I will be purchasing a new engine. Cylinder was ok, believe it or not. Anyone have an engine that they want to sell me resonable. LOL

Thanks for the input.