I have a Chinese GMS 76 that overheats when run at full throttle for more than 3 or 4 minutes. I can run on about 1/2 throttle. Its in a Midget Mustang and has a cowl that has been cut out to allow air flow.

Perhaps you have a GMS? GMC is a US auto maker, not Chinese (at least right now)!

It sounds like your engine is overheating. You probably need to redirect the airflow over the cylinder, and allow 2 or 3 times the area for airflow outlet that you have for air inlet.

Thanks. I was looking for that gov't bailout $$, you're too sharp for me, ha.

Why do you say it overheats? What does it do? If it runs for 60 seconds and doesn't over heat, you're usually good to go. I suppose the engine could build heat slowly over the course of several minuets but that sounds curious to me.

Are you sure it's not going lean as it burns fuel off and is then getting hot? You don't have the needles completely peeked at the beginning of the 4 min run do you? If that is the case you have a "going lean" problem not a "getting hot" problem.

How new is the engine?

This is the second year I have flown it and probably has 20 flights on it. I broke it in last year running 1/2 throttle for 2 tanks of gas, then varying the throttle during those runs. It died on me during one of my first flights this year and when I got it down the engine was smoking. Now last week I took it to the field, it started fine and I ran it 1/2 throttle and flew it two tanks of gas. It ain't fun landing dead stick!

Can you run it on the ground at full rpms for a while and will it stall the same?

If so, can you see metal particles in the oil over the model?

Can you test flying at full throttle, but without the cowl?

Hi!
Pictures please!
What prop size?
Glow fuel?
Glow plug?

As jaka said, what prop and fuel. It does sound like it's overheating but if it going to overheat and shut down, it's likely to do it earlier than you describe.

Bill

Take the cowl off and try it again. That will at least narrow it down to. If you got any 0% nitro fuel or 5% give that a try.

It's also worth the trouble of inspecting the engine for any loose backplate or cylinder head bolts. Go over the crank case with an allen wrench and make sure everything is tight and sealed properly. If you run it in on the ground without a cowling, keep an eye out for any fuel leaking from the cylinder head or backplate.

The GMS engines I've flown before were terrific performers, but they didn't tend to stay bolted together as well as some other brands.