At 3/4 full a 35 degree nose down incline is enough to uncover the clunk and kill the engine.

By 45 degrees the clunk is probably already uncovered and sucking air.

This will happen in the air too and contrary to the popular old timer "myth", the fuel DOES NOT stick to the back of the tank in flight.

Sometimes a longer length of fuel tubing will help smooth things out during downlines.

Try this:

Fill your tank fully, then try the same test.

If the engine does not behave the same way when the tank is full then you know for a fact that the problem is not with the engine or it's needle settings... and the problem lies with the clunk being exposed, etc.