I have a TT pro 46 spinning a 12.25X3.75 APC prop on a 3D plane. When I do an upright loop it runs fine. If I do an outside loop when the plane reaches the top of the loop it looses all power and almost cuts off. When I finish the loop the engine speeds right back up. Also if I am attempting a waterfall or blender it will lose a little power in the process but not near as much as it does in an inverted loop.

Engine has about a half a gallon through it.

Kinda sounds like the clunk in the fuel tank isn't
following the fuel.
Jerry

Gringo:
Tank may be a little to high in the fuse. Flying inverted or outside loop makes it difficult to draw fuel from the lower tank. Also make sure it is as close to the firewall as practical.

Flypaper, it is impossible to move the fuel tank. It is where it has to be. It is also touching the firewall so it is as far forward as possible.

In the other 10 or so flights that it has this wasnt a problem.

Any other ideas of what it could be before I pull out the tank and check the clunk?

Why is this only happening in reverse G manuevers?

Gringo;
Still sounds like a fuel height difference between upright and inverted. Only things that might have changed are fuel mix a little leaner, try richening it a bit, and an airleak causing poorer fuel draw. Also check line from tank to muffler for airleak causing not enough tank pressure. Just trying to pinpoint things.

Sounds like a fuel draw problem. Make sure the exhaust pressure tap is not clogged. Loops and other high G manuevers expose fuel draw problems. Post a pic of your engine, tank, and plumbing if possible.