How tight are your loops, and how long can you fly inverted? Here is a list of causes:

Your tank may be too far back, down, or up. If it is too far up your engine could be flooding, and if it is too far back or down then it is leaning out and quitting.

If it is quitting on the downside of the loop, its running too rich. If it is quitting on the upside of the loop, it is running to lean.



Give me more specificis on where in the loop it is quitting, what other aerobatics you can preform without it quitting, and how long you can fly inverted. It's s simple problem, I just need more info.

Sounds like your fuel pickup line is too stiff to "clunk" to the front of the tank. It has no problem going top to bottom, but it would need to double back on itself to go to the front. Take the tank out and hold it in a "dive" and see where the clunk is, I bet it is sitting at the back of the tank, all the way at the top.

This seems pretty common, as the most popular silicone fuel lines are really pretty thick. You may want to try to find some super-thin silicone fuel line and/or a heavier clunk. I have never been able to find good flexible fuel line at any LHS unless it was part of a tank kit already.

Originally I had posted a response about the Tower mufflers, but that problem (not enough backpressure) would manifest at the upside of the loop. This turned out to be because the Tower muffler has no baffles and was not creating enough pressure in the tank to feed the carb in a steep power-on climb. I fixed my problem by switching to an O.S. muffler.

That might be the case during a wingover or something, but in a loop, the plane has positive g's on it on the upside and downside, so fuel is going to the bottom of the tank, and the clunk is too.

During a loop, your primary experienced forces are lift (forcing fuel and klunk to the bottom of the tank) and gravity (forcing your fuel and klunk towards the dirt), in the dive portion of your loop, the gravity vector and the lift vector combine to put your fuel in the front and bottom of your tank, leaving a big airspace in the back/top. Lift force will depend on how tight your loop is and airspeed. If the klunk line is too stiff and you are not generating enough forces, you can and will suck air here, BUT NOT WITH A TOTALLY FULL TANK. That may be an easier test, fill the tank totally full, and do a loop as soon as you can.

Anyway, its an easy thing to check, I was just offering a possible solution.