Except it doesn't occur for quite a while, and the symptoms are of a RICH condition once the engine is been on for some time.

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...

If, after all that has been suggested above, your engine doesn't perk up, it may be worth un-mounting it and checking the backplate screws for tightness. Loose backplates can cause many strange problems that are difficult to diagnose.

Really, tho, I think if you replace your fuel lines and verify that the carb is clean and retuned from the start, it should work out.

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.

Sorry for all of the misunderstanding, and once again thanks so much for all of the help. I found last night that the fuel lines are good, and even tested the line between the high-speed, and the carb..fine. It has to be the low-end, we are currently adjusting the low end needle now, (which we found was screwed well into the carb, which would I guess result in more fuel than air or a rich condition. We have leaned the screw by 2 turns and are now seeing some improvement. I am letting the engine cool now so we can do another test run. It ran on full throttle enough for me to finally tune the high end needle but died from what I guess would be overheating. I pulled the plug to check and see if it was still ok after the dying of the engine, and it was and by the way very clean. I will let you know how the rest goes!

On my LA .46 I had to fully turn OUT the screw until the bleed hole was completely uncovered.

Even then the low end was far too rich, so I ended up doing as suggested, and slightly enlargening the hole.

This permitted the engine to run reliably, but the engine is still too rich in the low end.

I left it this way so as not to risk ruining the carb.

I'll bet that if you turn it out as far as you can, it will be much better.

If you do this, take out the screw and add a bit of locktight to the threads, so that you don't end up loosing the screw with engine vibration.