Sorry for being such a dork, but what is a DP kit? I think you have a fuel delivery issue somewhere along the way, or even more basic could be a bad plug. I would probably change the plugs first, if it persists the lines and cylinder heads should be thoroughly checked, but you are losing power not from too much air but not enough fuel. The popping is an indicator of that. I am not sure where you are located, but if you are in the new york area I will strongly suggest formula 1 in bayshore or queens, or cycleconnection in north jersey. However it may be, I am pretty confident without speaking for most others with an open airbox, that this is not an airbox situation.