my guess - main jet to large. I put a new carb on my Towny with following results:
so here's what jetting can do - I put the vm18 on my towny. With 92.5 jet it did 31 mph. Dropped to an 87.5 and it did 33 mph. dropped to an 82.5 and it did 36 mph. dropped to an 80 and it did 37. I don't have a 77.5 but that would probably be perfect and I bet I'd get 38 mph out of it.
7 mph just in jetting.