OK, story time so you know whats going on.
Got a derbi- stock 30mph
Added Tomos Tecno Estoril and, 13mm SHA, upjetted- 36mph
Full rebuild- still 36mph (thats good)
Ported cylinder- 32mph, less torque, but can really wind out downhill and on flats.
65cc Parmakit- 36mph, same torque as stock, but still 36mph
I have played with an assortment of variator weights as well
I have jetted correctly, tried different timing and so on. I know people who have stock cylinder ones that are pretty fast, I have kitted quite a few bikes before, using 13mm SHA's and got ~45mph out of them, but I am at a loss with this bike. It just doesn't want to go faster.