Sounds like a classic clogged carb. Get a spray can of carb cleaner and make sure it has the small plastic hose taped to the side of the can before you leave the autoparts store. Remove all of the rubber from the carb and spray it down paying very close attention to the pilot jet passages both on the inlet side of the carb and under the pilot jet itself. Make sure of the needle setpoint if it's adjustable (don't have a bayou 220 so not really sure) because that surely will cause it to lack acceleration.
Did the "expert friend" do a compression test on the quad? That would tell you (and the owner) alot about the condition of the rings/piston/cylinder/headgasket and would diagnose head gasket failure.
No compression test=no diagnosis.
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