Please help my yamaha jog 49cc

I had lighter rollers and new belt fitted today as my acceleration was very bad i was told this would fix problem. Yes i have a little better acceleration but now i cant go over 30mph and cant be restricted as it went 40mph before the fix im baffeled to what is wrong and so is the mechanic i think

 

Rollers too light. Engine not powerful enough to throw them all the way out to achieve top speed.

 

They were only half a gram lighter could they make that much difference the mechanic thinks there is a restrictor in exhaust but i dont think ao as i got to 40mph before i think he fitting bigger jets today as well its like its not going to second phase after pull off

 

Half a gram per roller equals 3 grams total weight lost.
Try heavier rollers before ripping the carb out.