It's kinda an overlooked thing but the air cleaner can make a huge difference if it has been neglected for a while. I have been caught by this one before. I popped it off and ran down the street with it off and it was a whole new quad. The air filter was two years old. I bought a new K&N and clean it now. Which reminds me........

 

yep it could be the muffler, or here it goes, carb not set right dirty leaky needle valve, clutches inspect the drive for worn bushings pins rollers weights, check the driven for broken or worn buttons and definetely check the bushings in driven that will defenitely make it be boggy, belt, the belt deflection should be about 1 in to 1.5 in, like the other guys said air filter and pre filter, spark plug maybe just starting to foul,
check the facts, Fuel Air Compression Timing
On a lighter note you may want to check the kiniffer pin as it may not be sliding in wobbulator shaft very nicely or may be low on tail light fluid[img]i/expressions/face-icon-small-smile.gif[/img]

 

You should see mine after I wash it, it actually has taillight fluid till I drain it out.[img]i/expressions/face-icon-small-wink.gif[/img]