The problem is your going high rpm's. The air is moveing incrediably fast. Then your slamming the throttle shut. Which causes the air to slow down drasticly. So your running way, way, way, way to rich. The gas is going into the motor and flooding it. The popping is the gass in the exhaust exploding and being burned off do to the heat. There is so much gas that you cant burn it all so it's going into the exhaust and your plug is ending up that way. You need to get spec's. Put it all back to stock jets and go from there. Your accelerator pump might be malfunctioning to. Check the overflow hoses on the carb. If one is blocked it will cause this. Check to make sure that the valve that lets gas into the bowel seal's propperly. Make sure your floats are adjusted right. The reason why the rpms increas when you pull the hot start is because it leans out the mix. Hope this helps let me know.