I don't think the QT has a second gear. I also have a QT and from What i can see its only got one speed. Mine kinda shutters at full speed and will back off slightly. If i let off and back on it accelerates again to about 28 mph and then backs off and shuttersa again.. But I'm thinking a carb adjsutment might fix that. I did a slight one but have not run it yet to full throttle I only did it at idle to get it to run smooth there. As soon as i get my new air filter This week I'll let you know. My carb is totally clean and so is my muffler so i know its none of those. Is this the same "upper end" type issue you were talking about?
I'm also thiking it might be my fuel flow from the tank too. When i had another tank on it for testing i didn't have that problem, right now the fuel seems to trickle out rather than run out. Check your fuel shut of and make sure its not plugged.