Disconnect (or loosen) an exhaust pipe at one of the manifold flanges. If it accelerates normally, find the exhaust restriction. (Maybe a squirrel has been using the muffler to store his winter supply of acorns?) Don't laugh- - - -I've seen it happen!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!