spark plugs heat range does one thing.. At high speed, a plug's tip should stay hot enough to burn away all deposits that collected on it during low speed driving.
If your timing is close, and you still get a true backfire (backwards through the carb) you may have a mechanical problem ..
Have you guys removed the cylinder head and taken a look inside? Have you examined the exhaust port and muffler to see if it's clogged with carbon?
A chunk of carbon in the head can get really hot and remain red hot. This glowing hot thing can ignite fuel at any time, regardless of your "ignition timing".. maybe it will ignite when the piston is at the bottom and the transfer ports are open, as fresh fuel enters.
Im not sure if a clogged exhaust will do it. Logically, it seems that if hot exhaust cannot escape through the exhaust port fast enough, some of it will be pushed down through the open transfers, into the crankcase and back out the carburetor ..