Really need more information. Like how enclosed is the engine? Where is the center line of the fuel tank with relation to the motor? Are you using the right glow plug? Try a new glow plug bad glow plugs can do some strange things. Is the fuel foaming? Do you have the fuel tank padded? How about the clunk? Have you made any hard landings? Like nose into the weeds. If so check the clunk it can get forced to the front of the tank and lodged there. How large is your tank? What size prop are you using. Try a different size? Have you tried a richer or leaner?