My Hangar 9 alpha trainer started stalling today after burning about 1/3 tank of fuel. I leaned it out one half turn and it hasn't stalled after that. However, I noticed that the exhuast had a sooty gray color for the first time. Am I overheating the engine, and if so, what are the symptoms of an engine running too hot? Thankyou very much in advance for any help.

Plowboy, If you can see a gray trail coming out of the motor while running then you are not too lean. That is a good sign that you are not running lean. The engine may have been running too rich before you adjusted it, you now have it in the sweet spot which produces good RPM, but is not running lean. If it was too lean, there would not be a noticeable smoke trail coming from the engine. Signs of overheating a engine would be to have Pre-detonation (a tapping or knocking sound) would come from the motor if it were too lean (overheating). Pre-detonation is when the piston is coming up on the compression stroke, the fuel will explode early due to the extra heat created from friction and lack of lubrication on the cylinder wall. The cylinder will expand and seize against the sleeve (Melting the cylinder to the cylinder wall). In pre-detonation the expanding gases will push down on the piston before TDC (top dead center). This is when the engine will start working against it’s self. If the explosion is too soon, the piston has to work harder to reach the top. The sound you hear is a pinging or rattling type noise usually just before the spinner and prop start to fly off. Pre-detonation is very bad...

Hope this helps,

DTB's example is of EXTREME lean conditions. Less lean can still be too lean though.

Try the "pinch" test. With the engine running full throttle, (carefully) squeeze and hold the fuel line feeding the carb. If the engine speeds up a little, then quits, it's set about right. If it just quits, it's too lean.

The other test involves lifting the nose of the airplane to nearly vertical with engine running full throttle. The engine should again speed up a little when the nose is raised, then drop back down when you level the plane. If you can't hear the engine speed up, it's too lean.
Dennis-

Where is the center of the tank with respect to the carburetor? While it may have been too rich before, your fuel feed may be a problem. Your engine should not die because it has consumed 2/3 of the fuel in the tank. I would try the pinch test with a full tank and one that is 1/3 empty. The response should be the same. The engine should speed up when the line is pinched.

Thanks for the responses so far. I want to clarify one thing a little. Previously, the slobber on the wing from running the engine was always clear. Yesterday, the slobber was sooty colored. It amost reminded me of the appearance of superfine ground engine parts (hope not though). Motor has good compression and runs strong.

A little "sooty color" in the oil residue is normal for a motor that is running correctly so this won't really tell you anything. In fact if the oil residue is perfectly clear then you are probably too rich. Use the checks that the other folks have suggested and don't worry about the oil residue color.

If you lean the motor further does it speed up, stay the same or slow down? As long as you are at least 1/4 turn or so from the point where it begins to slow down then you should be fine.

I would also make sure that all your connections and screws are still tight. Loose connections can leave a black residue everwhere.

As Geistware said, black gunk coming from the engine is a bad sign, a bit of a brownish color is normal. The black stuff is usually metal particles, and it means something is eating something else. The most common source of this is the muffler tail cone starting to losen up. I've also seen it from some of the bolts or screws on the carb.

An an ABC engine, the very first sympton of overheating in the air is power loss. The engine just slows down a tad from where it was when you took off. So if you take off with plenty of power, and notice the power reducing when flying, you are too lean, and you should land and richen the mixture.