If the engine is dying on down lines or inverted flight, its probably just starved for fuel. I've seen this before. People want a nice clean installation, so they make the fuel supply line very short. When the clunk comes out of the fuel in the tank, the only fuel available is the fuel in the line. Try putting a loop in the supply line between the tank and the needle valve. This will give you a longer reserve. Also make sure you're reducing your throttle on the down lines.

Also check your low speed mixture. You may be too rich on the low end. When you go inverted or on downlines, the increased fuel supply pressure further richens your low end, which may be causing the engine to load up and dead stick.

Finally, you might want to try the setup in the tank where the vent line has a clunk as well. This supposedly (I've never tried it) results in a more even fuel supply pressure.

Brad