A couple of questions for you:
1) Does this backfire happen at a steady RPM, as your slowing increasing the RPM or When you're going to WOT quickly? Different times will have different causes.
2) How large is the camshaft in the engine?
3) How much initial, and total ignition timing are your running?
Normally a backfire through the carburetor is an indication of being too lean. It sounds like you've got a couple of things going on here, rich at idle and lean off idle? You may not have the proper advance curve in the ignition system, it can cause a situation like this.