c90e engine

hi to every body. i have a c90e 6volt lump in my st70 (69 model) and she is running fine with a c90 carb. my top speed is currently just over 40mph. is this about right on standard carb/sprockets. the bike accelerates well through first and second and tails off a bit in top (to be expected i suppose)......any bright ideas on squeezing a bit more mph? the exhaust is a honda item stamped st50-st70. it starts first time every time and the timing seems to be correct. plug is a nice biscuit brown so i assume she is running about right. any help gratefully recieved and roll on summer!!!

40mph is indeed somewhat too slow for a decent C90 engine.
You could try a slightly bigger front (1teeth) sprocket or smaller rear sprocket.

I don't know the exact technical data off the C90 head and carb, so can't comment on any possible upgrades.

The original exhaust is somewhat restricting for any engine above 72cc.