I bet he's too rich.
Don't forget people I wrote that I went through the same problem a few days prior. If he sets his mid-range like I suggested it will solve a lot of issues. I also agree with Edwin about the plumbing. And Villa about the back pressure, and Rodney about the clunk. Check everything!
Also tighten down all your head allen bolts, check for any bolts or screws loose on your engine. Any air leaks will alter the way your engine runs. Check, check and if you're not sure re-check.
Remember people, the guy Posted that he's tried adjusting his high end while holding the plane straight up. That tells me that his mid-range and high end is off on the rich side as a result- over flooding his engine until he gets a glow plug drown out. Or he has plumbing issues on top of it all.
Winner takes all if he finds the problem and gets back to us. I'll admit if I'm wrong, I promise. Just don't tell my wife.
Pete