Backfires randomly through the carb and exhaust seems like a distributor problem. Are you sure it is firing at 4 degrees btdc? Maybe the valve clearance wasn't set properly? Can you see the accelerator pump squirt fuel, if you look in the carb, when you throttle it? Is it a new or rebuilt motor? Maybe the timing belt is on wrong.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.