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

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

Check for vacuum leaks. Spay starter fluid around intake,carb base, etc., while engine is idling, listen for change in engine idle.
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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 your idle/slow speed carb circuit is plugged. A carb cleaning and spraying all passageways with aerosol carb cleaner should find your lost idle

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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.

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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

try cleaning the carb.

broken throwout clutch springs?