ORIGINAL: vegas mossie
for the cost of fuel tubing, replace all the lines. even the one in the tank. while your in there check to see if the clunk has come loose or is stuck forward. check the stopper in the tank for leaks as well as the pressure fitting. make sure it is not pinched or plugged. if none of that solves it, run the engine up to temp, and see if the carb gets loose. you may also want to check your throttle connection, see how much vibration you have. good luck and let us know what you find...
The engine runs flawlessly the first time he flies it, for about the first 3/4 tank. There-after even if the tank is re-filled, he has this problem.
The clunk is NOT "stuck".
Since running the engine down to 3/4 of a tank takes more than 2 minutes ( which is about what is required to fully warm up the crankcase of an engine in flight ), this is unlikely to also be an issue with the carb getting loose, though it's a remote possibility.
The problem takes long enough to develop that this is more likely a issue with something that takes far longer to warm up, e.g. the fuel tubing or something surrounding the engine.
A leak should manifest problems in the first 5 minutes.