If you're running un-pumped, you may wish to try larger fuel line. The O.S. Max 1.60 is a thirsty engine, and regular line just may have too much fuel-line drag. You'll have to enlarge the fuel tubing from the clunk forward. We use a Sullivan tank and go up one size on the brass tubing, drill out the clunk to the larger inside diameter, and wire-tie all the silicon tubing to the brass.

Also use the larger tubing for the vent/pressure line.

Make sure the centerline of the fuel tank is at the same level as the center of the carburetor or no more than about 3/8" lower.

If you're running pumped, you do not use muffler pressure. The oscillating pump doesn't regulate, it just delivers, so you can get a good 'set' for full-throttle running, but it may be delivering too much fuel at other throttle settings. If you cannot close-off the fuel feed with the high-speed needle valve, then you have a problem with the needle or the needle seat. You should be able to get the engine properly-leaned at full throttle.