Myself and friends spent a frustrating afternoon with an OS61FX, that just would not run properly. Now we have tuned many an engine over the years, and after doing all the usual inspections and fixes, the motor still did not cooperate. It would run well at first from a cold start, but as it came up to temp', the RPMs would fall off dramatically. Idle was unreliable below 3200 RPM. The fuel is fresh...even tried a separate new jug..cool power 15%, the fuel system is leak free, and no air bubbles are in the line. Prior to our "tuning project", the owner had the usual OS61FX air leak problem, where the brass plug on the remote needle assy would loosen and back out enough to admit air, and so lean runs surely had occurred before we began playing with it. When cold, the motor felt like it had compression, but after running up to temp, the motor could be turned over without much of any compression resistance being felt. We have our suspicions, but rather than put words in your mouth...we'll ask the question.....whats your opinion as to diagnosing and correcting this problem?

Two quick things pop into mind reading this;

One, try a new glow plug for idle reliability check.

Two, have you checked the head bolts to make sure they are all tight?

several new good quality plugs were tried, and all bolts are nice and tight.

Sounds like those lean runs may have "put a hurtin''
on the motor...read wear and tear. When the engine
gets hot everything expands...but the clearances stay
correct due to the metalurgy. I suspect in your case,
the engine gets too loose, due to the wear, and the
loss of compression becomes apparant.

It would follow that the crummy idle is a result of the
lack of compression also. If this is the case....the only
cure would be a new piston and liner....

You may have an air leak elsewhere in the remote needle valve assembly. I had a lot of trouble with my OS .91FX until I finally went to a K&B remote needle valve assembly. This solved my leaning out in the air problem.

As for compression, it's supposed ot be that way. ABC type engines have a tapered brass sleeve that has greater expansion than the silicon-doped aluminum piston. When hot, the engine loosens up. ABC type engines are not known to be good hot restarting engines when flipped. When the engine cools down, the tight compression comes back.

When Super Tigre invented the ABC technology, it was for control line speed engines and was specifically to keep them from seizing up during a lean, hot run.

Some final thoughts on the OS needle problem. I have heard the leaning out problem described as an air leak, as being caused by vibration since the needle is attached to the back plate and because heat from the engine vaporizes the fuel. I don't know for sure what it is. A remote, not on the engine, needle assembly cured mine. I have also read in this forum that the plastic needle assembly from OS also cures the problem.

"The usual 61FX air leak"???

Never had it and never heard of it. Every 61FX I ever had and ever knew of ran perfectly right out of the box. We're talking approx 25 61FX's over the past few years at two clubs. Not one had any air leaks or needle valve problems - period.

The symptoms you speak of sound like you've perhpaps peeled the liner from too many previous lean runs. It can happen. Pull the head, wipe out the inside of the liner and take a look.

Try using a lower nitro fuel. Less nitro will run cooler, resulting in less expansion of the cylinder/piston. This should help give you better compression at running temp. If it doesn't work, your only choice is a new liner and piston.

Cool 15% is fine for that engine. I've been using it for years in everything but I add 4 ounces of Klotz synthetic to each gallon. 15% Nitro is not too high for a 61FX.