there are no rev limiters of any kind.. The PA50II is capable of much higher speed if a performance exhaust is installed.. i dunno about the Pa50I but probably similar.
If removing the muffler didnt change anything then yeah.. look at carburation. Get the emulsion tube out and re-clean everything. (The tiniest fleck of junk floating around in the float bowl can and will block fuel by clogging the main jet orifice)
The symptom seems to point at an extreme rich-or-lean condition.. bad enough to cripple the engine.. If all else is in fair shape something is radically wrong with the air:fuel mixture.