bought above machine after a 1989 of same make , model. wish i had keep old one. new one will not perform near what the old one did. 15 to 20 miles an hour in speed ,going down hill and on streight. difference is the carb i has a mikiuni BST31SS .old one had a mikuni VM24SS am thinking and want to replace with a bigger throated carb.am i thinking along the right lines or way off?