this baffles me as well, my qt50 used to toot along at over 30mph bu now i`m lucky if i get mph out of it !, up to now i`ve done the following :-
put new exhaust on
stripped cylinder replaced piston rings
replaced all gaskets
replaced spark plug
topped up battery with water
and you guessed it still no more than 20 mph !!
I think problem could now be in electrics somewhere, I just can`t get the flywheel off to get at them, does the qt have a set of points that can be adjusted ??
if you find out anymore please post, i`m not giving up on this one, i`ll cure it if it kills me !!!
John (Great Britain)