I'm coming back from about 20 plus years away from building and flying RC aircraft. During the hiatus I was able to hang on to one of my planes powered by an OS 40. The prop still turns with a little "gummy" feeling but there's still compression. I've pulled the head screws and was able to remove the head/cylendar sleeve and it all looks pretty good other than the carbon deposits on the top of the piston. The throttle is stuck so I think I need to soak it in something to get that free enough to disassemble. What's the best cours of action to get this enging back on line?

rinse the whole thing in lacquer thinner. Shake the lacquer thinner out and go fly.

Denis

Put the whole thing in a ziplock bag (I use freezer bags for soaking parts) and put enough model engine fuel in it to cover the parts. Leave it set for several hours to overnight and it should be loose enough to get everything moving and ready to run. If the bearings feel gritty, you might consider changing them before running it.

Yeah, old fuel works well for dissolving the goo, but if it's been sat for long enough, I'd say the bearings would have corroded, especially if it was just stored without any preventative measures. I usually knock the head and blackplate off so the fuel can get in everywhere, and when you want, it can get out again easily. Once it's all moving freely again, have a look at the rear bearing (easy to see) if it looks nice and feels nice, go play. If it looks rusty and feels like it's got sand in it, replace both.

One final thing, before you run it up (and i've seen/heard too many people NOT do this) flush your carb out to remove any gums, bits of fluff or debris. It should be SPOTLESS before you even think about firing that 40 up.

Hi mate,
"gummy" feeling? As in castor oil gum? I have always used kerosene as a solvent for this one.

And is the engine a FP 40? If it is then it is a plain bearing engine and little heat along with the kero flush should see it back in action.
(It was probably not the best idea to disturb the liner's position as it will never go back in exactly where it came from, but anyway...)

Good luck.

Thanks all, I have a good idea how to start now.

Recycled flyer, you said ". . . It was probably not the best idea to disturb the liner's position . . ."

The liner came off with the head. I'm not sure but it appears as if it's an integrated head/sleve. Anyway, the head is keyed and I doubt I will be able to reassemble it misaligned by much more than a half mm. Nonetheless, the deed is done and I'll have to live with it or be in the market for a replacement.

Hi!
Belive me ...nothing will remove/disolve old gummy castor oil inside an engine! And I mean totally remove if its orange or yellow! The best way to free an engine is to heat it up and oil it and go fly! But ...remember that after 20 years hiatus the ball bearings (if the enginehave those( need replacement due to all the old congelled castor oil that is stuck inside the ball race.