Help! I'm having a problem with an older OS61FS. It is mounted in a Top Flite SE5a and is fully enclosed with lots of in and out venting. I have flown this plane and engine combo for over 10 years and it had been very reliable. However, last summer on a typical day and typical flight the 61 abruptly quit. After landing, I inspected the engine, could not find a thing wrong, and tried to restart. Turned over nicely but would not fire except for a pop or two when starter was first applied. It had fuel, "spark", compression was good, valve timing and gaps good. I replaced the glow plug, no result, blew out fuel line at carb, check tank, etc. Still no start, except for occasional pop at beginning. I check intake o ring-ok, check for loose head, and all other obvious things. No obvious problems. I thought of a overheating condition, but engine was not unusually hot. Anyway, I finally got the motor started by cranking it over with the electric starter for an abnormally long time. Blockage in carb? I had checked and cleaned but maybe I missed it. Thinking that I had it resolved, I took the SE5a up for a flight. It ran well except it would drop rpm's to about half throttle for a minute or two and then it would throttle up again. Very inconsistent. The 61 reacted as if it was over heating. I removed the cowling and tried it. It would run more consistently but it still would act up. I used eli fuel (more oil) and with a noticable lack of power, it would run at a consistant RPM. Cooling or lube? I had changed the bearings about 3 years ago and there isn't any bearing noise. Compression seems good. I'm using a 12-6 prop and Cool-power fuel 4 cycle. I tried a different tank. The throttle arm is not moving when the rpm drop happens. It sounds just like a twin cylinder engine when it loses one cylinder. I really like this engine(I've got two of them) and really would like to get it running again. Any ideas on the problem or cure would greatly be appreciated.
Thankyou
Curt Spicer

It sounds like the compression seal of your engine is starting to go. While the engine may feel like it has "good" compression when it's propped on the ground, in the air the engine's heating up and the parts are expanding enough that the engine's losing compression. Likely replacement of the ring, and possibly the piston, too, will get back your engine's performance....unless the valve seals are also starting to go. With the amount of use you mention, it's not unlikely that the engine's just wearing out.