I have a Qt50 that I have completed a carburetor bore which required adding small amounts of material to the casting. Carb bore is now over 14mm opposed to just over 12mm stock. I've bored a stock 49cc jug (40mm stock) out to 44mm and installed a stock 44 mm Yamaha piston. I think the 49cc jug is a complete different set up and to obtain more power you need the YT60 jug. The reason being when you bore the stock 49cc jug you change port timing in the wrong direction and thereby inhibit full rpm/but at the same time increase displacement which can be deceiving. What I do know is that you can make a (weakly timed) Qt60 do a HARD 38 mph with rear gears and a 60cc set up. I've bought a digital tack and unfortunately have not set it up to read the rpm's at top speed but I realize that math does not lie. If your calculations are correct I"d like to think 50mph is do able with stock gears. The simple truth is I make this QT50 scream with PW50 gears and I don't need a tachometer to tell me it won't spin much faster. It will only do 38-40 mph. Mind you thats fast and you can cover a whole lot of ground in a quick hurry. But if anyone can make this engine do 50mph with stock gears I'd refer to them as the BAD A$$ MF'er I know and you don't.