I've got a little problem.
My OS 25 Fx was running great. Just recently it's started running badly. when it heats up, the power runs really low as if it's starved of fuel. This may happen after 7 mins of flying and the engine will even cut out!
I tried to lean it out, richen it up, but to no avail..could it be the glow plug? It's been in there a while. I'm using 10% nitro fuel, the same as I've been using all the time. I'm hoping it's nothing serious like the ring, but the compression seems fine.

thanks for your help

If the engine runs a while and then starts to sag after a few minutes, it's likely going lean for some reason. It's overheating and then sagging as the compression starts to go. A bad plug won't usually cause this. You should check for air leaks in the fuel system, dirt in the carb passages, a cracked clunk line, vibration-induced fuel foaming, and so on.

A very common problem is a cracked clunk line. The flex tubing gets a crack where it attaches to the hard tubing, and starts to suck air after the fuel level drops to that point.

As always, if you put the engine on a test stand and it operates "all day" as long as you keep feeding it fuel, then it's not the engine, but something in the engine/airframe/fuel system combination.

Thanks Bill,
I will give the system and inspection and let you know

cheers

Thanks fo ryour suggestion , bill.
However, I tried all that you suggested. I took the fuel tank out, pressure tested it and the lines. I also cleaned out the fuel filter...but the problem is still there. I may try some new fuel, but the other stuff I'm using is not so old.
Ant other suggestions? I don't know if I told you, but the engine is mounted upside down...its in a corsair.

thanks again

Is it giving problems when cowled or uncowled. If cowled, it may not be getting enough cooling. Air must go THROUGH the cooling fins. Having a big air inlet and a big air outlet don't do any good if the air can bypass the engine. You may need to baffle the engine to insure it gets good airflow through the fins.

Also, try it mounted upright on a test fixture. If it runs well, then it's the installation and not the engine.

It's a cowled engine and it has always been that way (say 6-8 months) andhas never had this problem. It has also alway been mounted upside down. I tried some new fuel yesterday, and that seemed a little better. I am going to try a new glow plug andsee how that goes. I know you said it shouldn't make a difference, but i\I'm clutching at straws now!I'll let you know. Thanks again

Tried the new plug, but as you said, made no difference. [&o] Could it be a compression problem? It seems alright, is there a specific test I can do? I thnk I once saw a little fuel leaking from between where the main bearing is housed and the spacer that locks the prop in place. I only saw it once...could this be the cause and can I fix it? cheers again

If the compression is starting to go, and you're leaning the engine to compensate, that could be a source, but lowered compression usually affects idle first. The idle and acceleration become irregular or unreliable. Some fuel leaking from behind the drive washer is normal. That's how the front bearing gets its lubrication. If you're getting enough fuel through there that a lot of raw fuel is wetting the aircraft, then the fit between the crankcase and crankshaft is shot. The seal is made by a film of oil between the two. The front bearing doesn't make the seal.

As I mentioned before, if you can mount the engine on a test fixture in the upright position and get everything set up "perfectly", do so. Testing the engine that way will allow you to determine if there's a problem with the engine. If there's a problem, it should show up on the test stand.

yep,
It was the compression. Finally went on the bench...now it's really hard to start and won't run. thanks for your help, I've just ordered a new sleeve and piston. Now I have to deal with being grounded!

cheers again