Just bought a Extreme 90 cc for my Son to drag race on dirt / light mud. The atv has been sitting for 4 years and has great top end but real slow getting there. Takes about 250 yards before it sounds like you hit Nitro and shoots to top speed. It gets beat by a Polaris 90 cc Sportsman and a 80 cc Yammy Bager with no mods on either quad.The Extreme has mods done to it as in , carbon fiber exhaust, UNI Filter, Keihin Racing Carb, Malossi Clutch and some engine mods. Can this be a jet in the carb clogged or maybe a weight sticking in the Variator?
Thanks for the help.

 

You've answered you own question.
Clean the carb and do clutch maint.
Go from there.