I have a 54 registration Yamaha Jog R 50cc, which a few months ago under warranty had a new carb and jets fitted because from day 1 when i bought it i was experienceing erracting running. Let me explain, i would kick start the engine, let the engine warm up (idle for 5-6 mins) then ride in a normal fashion.. When i opened her up (i had her de-restricted) she would accelerate very quickly to around 40mph..but then after a while she would just drop down to between 25 and 30 mph..This has been going on as i said since i bought the bike..and now when i pull away from stationary she takes forever just to get to 30 which i consider dangerous..can anyone help me ?