O.K. assuming that you have checked the radio settings and the ATV isn't limiting sevo travel, the best way I can see to fix the problem is to move the linkage position on the servo horn.
I would flip the little plastic bit that the linkage rod goes through over so the rod is on the engine side of the screw. That should give a little more travel at the carb. Or if there is a hole on the servo horn further out , use that or find a longer horn.
