I have this exact combination, BME 50 in the GP Wagstaff, and had the exact same problem as you. Turned out that after sitting for a while, the tubing inside the tank got so stiff the clunk wouldn't even move around anymore, and it was sucking air around where the tubing connects to the brass tube. So I replaced all the plumbing on the plane (just not to take any chance) and the engine ran fine afterwards.

Gabriel