I bought numerous 80cc kits and they all run very well, except one. It starts right up, revs up great without a load. I've ported out the exhaust pipe, installed an NGK plug, sealed up the carb and intake manifold, adjusted the carb metering rod every way possible, changed the carb and it still sputters all the time and has no power. All the kits came from the same place and all came at once. I cannot figure out why this one won't run. Any ideas? I'm ready to pull off the cylinder and see if there is any casting plugging some internal ports. Please help.