How did you run the engine on the test stand versus the airplane. is it upright in the test stand and inverted in the airplane?
As DirtyBird mentioned, it may be too rich inverted. You may need to check the fuel tank and fuel lines carefully too. It sounds like it is getting air bubbles in the fuel line from somewhere. Maybe the vibration is causing excess air bubbles to get picked up by the clunk. Or there is a subtle air leak of some kind.