rm 250 revs on its own

I've recently bought a 2002 rm 250 and was rebuilding it. I put a new top end in it, all new gaskets, etc. I goto start it and it will rev up on its own and drop back down. It will consistently do this to the point you can't ride it. I've checked the throttle cable, checked for any air leaks, it acts like just for a second the timing advances and the goes back to normal. I've gotten a new stator and coil, still does it, replaced almost everything except the cdi box. I've read about the rm requires a special plug, it has it in it now but when i got it it didn't. Could the wrong plug have messed something up? Any help would be much appreciated.