Intake backfire can come from timing problems, which you have investigated thoroughly. Make sure your ignition wires correspond with the proper firing order so no cylinder is firing before it is supposed to. This situation can also be a "lean pop" which happens when the slow burn of a lean mixture continues in the intake manifold rather than completing in the combustion chamber. This sounds like it might be your problem. Verify by adding a little choke to enrich the mixture before you quickly hit the throttle. if the problem diminishes or disappears, then you're on the right track. if you confirm you are too lean, then take steps to enrich the mixture. You might just need to clean the carb, adjust the mixture, or re-jet.
