Well, that was a big problem with those old a-3's. When this happens, just let of the gas for a sec and then get back on the gas.(you should have then shifted) As time progressis, if this bike has sat for a while this shaking problem will get better. To help with the problem for now, change the tranny fluid with no detergent 10w 30 or SAE 30. Do you know how to change the fluid in these mopeds?