49cc xinling scooter loss of power?

have twist and go scooter that used to reach 35mph comfortable,now i can only reach 20mph?checked filter,rollers,new drivebelt and new carb.have no problem moving off ,but cannot feel the stepup in automatic gear.at my wits end,the first bicycle that overtakes me, i am packing up.help please.

eddie slowman phillips
April 2009