Re: 43mph 48cc stock internals +- 1500 miles later
I rock a 48cc on a gt2-a frame. I weigh in at 200 lbs on a 6'2 frame. I clocked 43 as well on a flat surface. So far I have: (in order)
-tore down engine upon arrival to clean out metal shavings
-smoothed and matched the transfer ports
-Lapped cylinder head and replaced head gasket with a very thin layer of copper rtv (no leaks)
-homemade poopoo pipe (3/4" conduit w/ stock muffler at rear)
-re-jetted stock NT carb
With just that little bit of work, I consistently run 40+ mph. Very smooth and lots of power band. And that's on a 44t sprocket with the factory tensioner. If set up properly, it all works well. The important things to remember is that if it creates drag, it is slowing the engine down. Too much chain slack causes drag. Too little slack causes drag. Flat tires, misadjusted wheels, loose or tight bearings, lack of grease in the bearings, too much grease in the bearings, etc. All that stuff will make you lose speed.
I will say, though, that it's not about speed. Sure I can get there when I want, but I'm not going to keep my bike at that speed for more than a minute or two. That's a quick way to destroy that little chunk of melted down soda cans between your knees.