I run Super Heavy class , and the smaller driver seems to always pull better , especially on a dry slick track . I would do as one other stated , and stick with the driver you normally do , but go to a 58 , 59 rear gear and see what happens , you may be gear bound . The lap times will tell the difference . What do the rpm drops look like ? If he is on the gas like he says , there shouldn't be but a few hundred rpms dropped each lap . If the drops are over 800 rpms , he isn't on it , or the kart is to tight , or your stagger is off . I also like the smaller gears and driver , the chain seems to roll across the gears smoother , and they def don't get abused and bent by the surface or an odd ball rock that might have worked its way up .
Al , you really need to build you a 4 cycle engine and get it on a kart , so you can compare the apples and oranges . 2 cycle - High RPMS , no low end grunt , best when wound up and kept wound up . 4 cycle - better suited for low speed grunt , requiring bigger drivers to obtain the same speed as a 2 cycle that can turn a million more rpms than a 4 cycle . 4 cycle - easier to maintain and requires less fine tuning while on the track ( How many have seen a 2 cycle racer messing with his carb while trying to race ? ) Sorry , getting carried away ! Good Luck with your racing !!