I would say try a different brand of plugs I had a buddy build a brand new engine and this happened and it turns out some of the brand new plugs were bad. Switched them out and ran clean up to 7500rpm's. Also check the wires at night in the dark and see if they are arcing anywhere,I had that happen and the wires were not that old but near the headers there was lots of heat....Also I would check the carbs out they could be lean if it's popping cause it's lean you probably should be able to see the headers glow at night also if it has headers...