Make sure the fitting on the muffler isn't stopped up. It does sound like you are lean at top end. You did not say if at idle it is blubbering or smoking quite a bit. I would suspect that the low end (idle) may need to be leaned somewhat.

Turn the plane upside down and start it. Run it up and down and see if it runs differenltly than when started right side up. This may give you some clues. I have a feeling it is loading up while at idle and when started upside down this may not occur.

Jim