Honda Super Cub Motor?

I recently rebuilt a honda c70 motor, but when I clamped it on and adjusted the timing with a timing light and did the whole ten yards trying to get the timing just right I can only get a top speed of 35 miles an hour. Its rated to go 50mph. I checked my brakes and my carb jets, but I really don't know what is going on everything I do doesn't seem to change the top speed. Going through first and second the engine revs pretty fast and gets me to around 25 fast but then once you reach 35 it just stop reving fast and just stays at a constant low pace. Any ideas how to make it rev faster, go faster, or any ideas in general.
ps- valve clearance is correct.
 
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