Bought what I thought was a pretty clean 96 Xplorer 300 that needed a new top end. Didn't get the old piston, so no idea if it blew up before or was just worn. Of course the cylinder was maxed out already when I went to have it bored. New sleeve and boring back to stock with a new WSM piston, rings, and gaskets. Got it back together and it has an erratic idle, bogged under throttle, and backfires when you let off the throttle. It looked like the lower end had sat for years with fuel in it, so I replaced the crank seals and there was no change. Rebuilt the carb changing out the jets, needle, and needle and seat, still no change. Put 2 cans of carb cleaner through all the passages and cleaning. At my wits end with this thing. Anyone come across this type of issue before? Intake boot looked fine and doesn't change behavior when spraying with carb cleaner while running.

When I pull the plug, it is wet and blackish after 2-3 minutes and it is unrideable.