From the looks at how you are specifying "twin carb" like it is a big deal its not bro. Id be surprised if your engine was opened for anything but a rebuild. If you were not retarded and were looking for top speed you would have 4mil Cheetah minum THEN you could run 35's or so. Not to mention you have 20cc domes. If you are running 38's on a stock motor who cares if you are looking for more top you cant even get there when there is no bottom. You would need a run way the size of the fucking Pacific Ocean to even get 2nd gear. And to even consider 42's you have no fucking clue what you're talking about. So i would like to see a video of your Bike with your wimpy ass motor running on 38's.