Jeff, check the pressure nipple on the muffler,, make sure it ain't clogged or that the muffler bolt is closing up the hole when the muffler is bolted up to the case.

Other things to consider,,, is there a lot of fuel escaping between the venturi and the case? Is the spray bar properly positioned and/or is there a lot of fuel escaping out the front bearing while the engine is running?

BV