First thing I would try is draining the fuel and starting with fresh gas, you may just have some bad gas. I am assuming that the quad starts and idles fine, but as soon as you give it gas it runs like crap. If none of the other advise has helped, take the top off of the carb and look inside. Make sure that the spring, retainer and needle are all as they should be. I have seen problems with these carbs with the floating slide. The needle retainer breaks and slips up into the spring and doesn't hold the needle down anymore. So when you give it gas, the butterfly valve opens letting air throught the venturi, which lifts the slide valve. Since the needle may not be sitting in the proper position, it may be allowing the main jet to open too early, flooding the engine. If every thing looks ok in the top of the carb, take the float bowl off and make sure there is no debris in the carb pluggin things up.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.