Three guesses.
Either some of the tiny orifices in the carb got clogged.
(do you use an inline fuel filter and a capfull of some gas treatment occasionally?)
I swear ...I have to clean my PA-50 carb every 150 miles !
Or.... you sustained a seizure which damaged the rings and bore and caused a loss of compression.
Or.. the pipe is clogged.. and your straight pipe ain't cutting it.
(but I would think the straight pipe would perform better than a badly clogged stock muffler)
Could it be the 200 pound dog house you forgot to unchain it from ??.. ;)