Will idle & run at slow speeds, bogs down at higher speed.

You may need to do a carb clean and adjustment giving it a higher idle speed.

Sounds to me like your not getting enough fuel change fuel filter and tak gas out of tank and clean it out a few times as the screen in the tank may be clogged also if you have an electronic fuel pump make sure it is working properly

Harry, dirty carb or injectors.
Check the attached links,Manuals and guides, Good luck with your Bike
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ATV TROUBLESHOOTING AND MAINTENANCE
ATV Service and Repair Manuals
http://www.carlsalter.com/motorcycle-manuals.asp

Sounds like engine timing is way out mate take off right side engine cover and have a look at timing marks hope this helps

Sounds like your idle/slow speed carb circuit is plugged. A carb cleaning and spraying all passageways with aerosol carb cleaner should find your lost idle

clean the carburettor out.might be a blocked jet in there where the fuel passes through..it only takes a tiny piece of dirt to stop good gas flow.there sould be 2 jets in the carb,and they are usually made of brass.1 jet (for engine idle and low speed).the other is for when more power required.

Prairie 300 carb problems - YouTube

? 0:28
www.youtube.com/watch?v=w1-znueTqFE
Dec 3, 2012 - Uploaded by Joey Fortner
Prairie 300 carb problems ... The slide (the piece you see moving up and down) has the needle running .... 2005 Kawasaki PrairieIdle issue.

you might have a dirty carb.check the big jet in there as it might have a blockage.sounds like you have enough gas going through the small jet to start the quad and for slow speed but when you want to give it good power, it starts to bog down.do the carb thing first

broken throwout clutch springs?

First question since i am assuming it was just sitting around for some time if i am correct you need to pull the carb remove the float bowl and components if everything was done correctly with your rebuild the likely problem is your carb is gummed up remove everything in the carb low speed jet etc. soak everything in sea foam fuel cleaner overnight and hopefully you have some compressed air and blow out that low speed jet and passage way pay close attention that you have see fluid coming from the other end and check the jet make sure it is unclogged hold it up to the light.