Are you sure it is backfiring? If you are running too lean it will pop out of the carburetor which some people confuse with backfiring. The problem with running too lean is that you can damage the engine... maybe even melt a piston.
If it is backfiring you should be smelling unburnt fuel coming out the exhaust and have nasty oily plug.
If the plug isn't oily I would try upjetting a bit more and see what happens. Also check for restriction in the exhaust and fuel systems.