Hmmm. I just checked. Mine is 15/36. Maybe the valves need adjusting. I guy below said his with 15/38 tops out at about 36mph. I don't feel that bad now though as I did after reading other CT70's cruise at 40MPH?
36mph top speed is weak. IF your K0 engine still has the original 17/69 primary gears, then you're 5% overgeared. These engines don't make much torque and don't tolerate even slight over gearing. Easiest way to find out if if over-gearing is slowing you down is a 2-way banzai run; that will eliminate the effects of wind or/and grade. If the sprocket combo is holding you back, then top speed should be 5-10mph faster on one direction.
If valve clearances of big enough to cause power loss, the valvetrain should be really noisy, hard to ignore. Check point gap, if it's less than 0.012" it's costing you power. Worn chain & sprockets can create enough frictional losses to scrub-off a few mph from the top. How's the airbox & filter element? Is the rear (inlet side) rubber boot pinched? Is the foam new(ish) & clean? What exhaust are you running? Originals can sometimes become partially clogged, that's not very common...only possible. If you're running one of those Chinese knockoff mufflers, that's the restriction right there...those things are very restrictive on anything bigger than 49cc.
Gotta say, my first thought is low compression. That's worth verifying. Without full compression, you'll never get full power.
I could see 35mph sustained cruise speed on a stocker. But normal top speed should be 42mph+.
