Since you asked in a PM, I'm putting the info here, where others can benefit from it.
An air leak is a nightmare in any engine, and usually the last thing someone thinks of when it gets stubborn.
Take off the backplate, and apply a thin layer of red gasket silicone where it mates with the block. (any auto store has it) Instead of tightening it all the way, just snug it down by spinning the hexes with your fingers. Make sure it's seated flat, and the RTV is about the thickness of a couple of sheets of paper. 24 hours later, tighten the screws.
What this does is create an actual layer of gasket material, rather than squishing it all out by tightening it down right away.
For sealing the carb, check out the sticky on top of the General Nitro Talk forum. That one you need to push down hard on while tightening the clamp, but you can do it right away.