Kipper,
Here's some of the problems I've had and solved with my various R/C aircraft.
One thing to check with fuel tank mounting is to try to isolate the fuel tank from airframe vibration. My trainer would run fine on the ground (when airframe vibrations were damped out from the plane touching the ground), but inflight, the fuel tank would vibrate enough to make the fuel foam! It took about a year and 2 complete engine swaps to figure this out. I solved the problem by widening the one bulkhead that the fuel tank had to stick out through, then isolating the tank from the airframe with good quality latex foam rubber.
Make sure your fuel tank is pressurized from the exhaust, and make sure that the pressure tube inside the fuel tank is at the top and not submerged in the fuel making bubbles. You might consider moving the pressure tap on the muffler if you think you're not getting enough pressure. Also important is sealing the gap between the muffler and the engine to ensure the muffler is properly pressurized. A scrap of thin high-temp gasket works fine, so does high-temp silicone gasket sealer although that's a messy option.
Make darn sure you're using a high quality fuel filter that does not leak at all under any conditions. Check for air bubbles in the fuel line between the engine and fuel filter when the engine is running at high speed. This has to be the most common reason for fuel system inconsistencies I've seen. Before I pointed this one out, half of the guys in my old flying club were leaking air into the lines at the fuel filter.
Make sure you're using the right prop size so you're not overspeeding or bogging down the engine when you manuver.
Make sure if you have a fuel filler valve that it also does not leak any air. If it leaks or seeps fuel between trips to the flying site, chances are it also will leak air into the fuel lines during engine operation.
Make sure the fuel tank is sealed up when pressurized. You can check this by filling the tank with fuel, plugging the line that goes to the engine, and then pressurizing the tank from the exhaust line. There should be zero leakage from any fittings. If you use your mouth to pressurize the tank, be careful because when you release the pressure, it'll probably squirt fuel back at you (been there, done that, 15% nitro fuel tastes really bad)
Keep the lines from the fuel tank to the engine as short as possible.
Replace the fuel lines every few months to help prevent pinhole leaks. Fuel tubing is cheap...
Like HT said, don't run your engine at the perfect ground mixture. Find the best mixture on the ground, then tweak the needle valve a bit towards the rich side. Your engine will thank you. If you're getting a lot of discoloration on the engine, you're probably getting it too hot.
If you really can't get the engine running right, consider getting a fuel pump. There are some relatively cheap ones that operate off of exhaust pressure and provide a nice consistent fuel pressure.
One last thing, higher nitro isn't always best, and nitro content above 10 or 15% can make engine tuning difficult. YMMV here, it really depends on the engine.
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eagl <squealing Pigs> BYA
Oink Oink To War!!!
