ORIGINAL: jester_s1
I would suspect a bad clunk line in an engine that runs fine through half a tank and then suddenly quits. Either a hole in the line or slipped off the tube. If it's a hassle to get to the tank to check it, you can test the theory by seeing if the engine will run with only a little fuel in the tank.
You also didn't say what kind of planes you were flying. If the engines are in a cowl they may be overheating and shutting down, even if they are tuned right.
Good Point! Very true. It's funny but including myself I normally wouldn't think of that. But it once happened to me with my Hanger 9 Fokker DVII when I had a Evolution .61NX in it. You will notice that the engine is real hot, you can feel the heat radiating from the front of the engine. Also if you turn the prop by hand without the igniter attached, it will turn over tight. Plus if you were to feel the crankshaft it will be very hot.
I wound up making the air escape holes bigger under the cowl, and switching my engine over to O.S.
Also I switched my glow plug from a #8 to a #5 to solve my problem and it worked. O.S. seems to be able to take the heat better than Evolution, they have more power, way more dependable, and can take more abuse before any real damage occurs.
I have the new O.S.46AX and the engine is fantastic! Minnflyer did a write up on this engine and he's right on the money with his evaluation. Most all my engines are O.S. (4 out of 6)except for one and that is a Saito FS150GK new still in the box, and a new Super Tigre .51 that has a Perry Carb and Mac muffler.
Pete