Mysterious rev limit on gy6 50cc, replaced half the engine and no change

[rudyauction509]

I picked up a 50cc gy6 scooter a while back and it ran fine for a couple weeks then started hitting a rev limit like someone was shutting off the ignition when I reached a certain rpm and turning it back on when I slowed down. I've replaced the stator, CDI, ignition coil, spark plug, as well as the piston, cylinder, cylinder head and camshaft and I also replaced all the wiring between the stator and the spark plug. It has a big bore kit, performance exhaust and air filter and is running a bit rich. The problem usually only starts when the engine has been running for a while. I warmed it up and put it on the center stand, then removed the spark plug boot and held the wire on the spark plug while revving it, and moved it away so I could see the spark jump. The spark was still jumping 3/4 of an inch to the plug when the rev limit kicked in and moving the wire closer or farther from the plug does not effect the rpm the limit kicks in. I'm lost here and the only other thing I can think of is something in the carb that's suddenly cutting all fuel when I rev up. Also the rpm the limit kicks in goes down the longer I ride. Any ideas? I was able to get the exact same sound the engine makes when this rev limit kicks in by shutting the key off at full throttle but when it happens on its own the spark plug is still firing strong.