Yesterday I ran my new-to-me 120 Surpass (no pump) for the first time. It started up nicely and everything seemed OK on the ground. After flying it I noticed a lot of fuel on the cowling that seems to be coming from the needle valve. I remember reading somewhere this is a bad gasket or o-ring. Do I need to tear down the carb?

In the air, on the down side of a loop and afterwards, it sputters. Back on the ground I ran it full throttle and held it (carefully away from my toes!) nose down, and after 6 or 8 seconds, it quits. (It occurs to me this might not be such a giant issue, since who needs to run full throttle straight down. But I want to find out what the problem is.) What's going on here? Is it possible it's simply too rich? I tuned it with my tach for quite a while before flying it, and it's turning about 9000rpm with a 15x8 prop. Is the leaky needle valve causing this?

Also, how hot is too hot? It seemed hotter than I would have guessed, but I'm new to 4-strokes, so it also seemed noisier and had a lot more vibration than I would expect (I'm told by an old hand that the two latter features are normal). I'm going to buy an infra-red temp sensor, but what kind of temps should I be seeing on the outside of the engine? Can I add extra oil to my fuel to help cool it?

First off, if your engine only has a few runs on it, it's nowhere near broken-in. It will take over a gallon's worth of running for the engine to come in, starting out rich and working towards peak RPM.

Fuel will drip somewhat from the carb, especially at the lower throttle settings when muffler pressure is used. That happens with an updraft carburetor.

Holding the model nose down will cause the engine to go rich and then quit. It goes rich because gravity pulling down will increase the fuel pressure a bit. It will quit because the fuel pickup will become unported and you basically run out of fuel for the engine, as the fuel line draws only air.

As far as temperatures go, it's normal to the engine to get hot enough to cause a nasty burn. We don't worry at all about engine temps. We consider the RPM the engine's running, how it handles, and how it sounds. We never monitor temperatures, so we don't have any to give you.

Basically, as long as the engine's running rich of peak RPM and is handling well, and you're using a good-quality fuel, the temps will take care of themselves, assuming proper airflow to cool the engine...and this will be borne out in the handling.

Thanks Bax, for the very helpful and complete answer.

I'm actually unsure how much break-in time this engine has had. I'll start treating it as though it's new, as I should have from the beginning, and run it on the bench for a while before taking it back to the field.

OS suggests that this engine needs at least 18% oil in the fuel. I'm using Cool Power 4-stroke 15% nitro, which has 17% oil if my online reading is correct. Is this adequate for the engine? Sometimes 1% is insignificant, and sometimes it spells the difference between success and disaster.

I presume 9000rpm is normal for this engine on the bench with a 15x8 blade.

The Cool Power fuel is fine, though we usually use Morgan's Omega in all our engines...Omega is basically the same as Cool Power, except it has some castor oil as part of the lubricatioin instead of all-synthetic. Color's different, too, but color doesn't matter.