Had the exact same problem with a Saito .82 on a Venture 60. Played around with both needles (hi,low) with no success. Really didn't want to get into the tank, because it is quite a chore to get it out and back into the plane. Finally had to do it. I shortened the fuel line in the tank to give the clunk more space away from the back of the tank (about 3/4 inch). Apparently, the clunk was too close and while climbing for extended periods the fuel line would stretch just enough causing the clunk to basically suck up against the back of the tank preventing proper fuel flow. Haven't had the same issue since. I had it set up so I could get every last drop out of the tank, but I've decided that's not practical.

I'm interesting in seeing one of these. Anyone have any pictures?