well a backfire through the intake is kinda of a common thing, you probably have a rich mix, i believe, but again i would ask over at ecj5.
Do this, pull your plugs, clean them real good, then replace them, run the engine for two minutes, through all RPM's. let the engine cool down, take out the plugs and look at them, if they have lots of ash your runnning rich, actually they really shouldnt have any ash on them
that will tell you if your running rich, then i owuld put a ai cleaner on it, b/c you shouldnt run an engine wihthout one, and i assume you dont have one b/c then you wouldnt even notice the backfire.
once again not an expert at these carb adjustment and timing adjustment things so i would ask at ecj5 forum