my funstar with a tt50 heli engine and a heli muffler runs well upright and inverted, but not when the nose is pointing up. It slows down quick and quit. What could be the problem here? It idles very well too. The engine was previously in a seagull low wing trainer, it ran well, upright, verticle or inverted. Could it be because of the muffler? I was running it on the tt46 muffler previously in the seagull lw.

One of two problems. First, the fuel pickup (clunk) is stuck in the front of the tank and when the nose of the plane it up it can't pickup fuel any longer. Second, you have the motor running too lean. When you raise the nose it leans out even further and dies. Try richening up the engine a little bit. A couple of click rich. You should be able to see just a little bit of smoke in the exhaust when you have it rich enough.

Hope this helps

Ken

I'd look at everything Ken said and one more thing. While you are looking at the clunk, make sure it is NOT hitting the back of the tank. It should be roughly 1/4 inch from the back of the tank

Bubba,
Thanks for covering my 6 on that one. Yes, what he said!!!!

Ken

i tried running the engine richer(15clicks) and there isn't much difference. Other than the clunk line, could there be other problems? I am quite certain that there is no problem with the clunk, but in any case, I will take out the tank and try.

Air leak in the fuel system/carb could also cause this

small pinhole in the pickup fuel line inside of the tank--most likely near the brass tubing--that only sucks air and becomes evident when the plane is in an upright attitude, and the fuel runs to the bottom of the tank, exposing the hole to air. At all other attitudes, the hole is covered with fuel, and is causing no problems. Does this also occur in the normal, straight and level attitude, once the fuel gets below half a tank, or have you gotten that far with the fuel yet?

Just a thought...

it sounds to me like 2slow might be right on the money here, if everything else has been checked. While you have the tank removed, you might as well replace the tubing connected to the clunk, can't hurt. I don't think that there is anything different with a heli engine that would make it act as described. It's usually these minute details that can cause the most headaches. good luck.

well, the truth is out. It wasn't the heli engine, it wasn't the fuel tank or lines and it wasn't the 135degree mounting. The problem was caused by the heli muffler. I changed the muffler back to the tt46 muffler and all is well again. BUT, I LOST a LOT of POWER. I think I lose at least 1000rpm and the engine is actually warmer with the airplane muffler. Any idea why a heli muffler cannot work in plane?

The back pressure created by a muffler is what is used to pressurize the tank. Sounds like the heli muffler doesn't create enough back pressure.

you may be right piper, since the heli muffler has a much larger volumn and it make sense that when verticle, there is not enough pressure to push the fuel against gravity. But what I don't understand is that why it can pressurise a 330cc heli tank, but can't do so to a 8oz funstar tank. Could it be that my fuel tank(arf fuel tank) is too soft, resulting in the tank expanding instead of pushing the fuel when it is working against gravity? This coupled with the fact that the heli muffler gives less pressure results in no fuel feed to engine?

Is there a header tank in the heli that the engine came from? Also the distance of the tank may be just a tad further away from the mufflers pressure tap in the airplane that the heli so the difference may be just "over the line" enough to be the cause.