Im not sure, but it may be a bit much for the bike, being around 300 pounds, but then again it might not, my sister (125 pounds) and her husband (175 pounds) went on my xt250 for a spin, and it did fine with both of them on board.

Do you have anyone who is average weigh around you that could take it out for a good test ride? lets say around 180 pounds or so, that way if they ride it and it goes to about 70+mph (top speed) then you know there is nothing wrong with it, and if it still does the same thing (53mph top speed), then there is definately still a problem.

Thats the first thing i would do.

Stock XT225