ok make sure the clunk is where it is supposed to be and hasn't shifted around and stuck somewhere high in the tank. do you see any fuel leaks you may be losing head pressure from the muffler as the attitude goes nose down. lastly replace the fuel line to the clunk its covered by fuel nose high so no leak but as the attitude goes nose down it may expose a pin hole causing a leak back into the tank and fuel starve your engine. as far as the prime goes try this open the throttle all the way put your thumb over the carb and rotate the prop in the direction of rotation until you see fuel just touch the carb and start normally. be sure to close the throttle again before you start.
let us know what you find