I thought I had that exact problem, but it turned out to be a fuel blockage which allowed a smooth idle, but bogged terribly when I turned the throttle, making me think it was stuck in second gear. If was only after much work and searching that I accidently discovered the real cause. First, try cleaning your carb's fueld filter, and if that doesn't work empty your gas tank, take out the filter if one is there, give your tank a little flush, clean your filter well, taking it apart to clean if possible. Clean your carb's fuel filter again after this. Oh .. the blockage might also be in your jet, or in your spray bar, if your carb has a spray bar.