Zip set up

So I have a 2 stroke zip with a pm 70's kit and a pm 360 exhaust with mallosi 6.5g rollers and a 64mm main jet.
When riding it struggles to get up to 40mph, but when it hits about 43mph it will quickly speed up to 50+.
How can I get it to 40 mph quicker, it's it just simply changing the belt and springs or is it more complicated

 

try lighter rollers, bigger jet, variator shims

 

I did have 5g rollers in but it was reving to high. What size jet would you recommend

 

id still guess slightly lighter on your rollers.5.5's it sounds like your going up your transmission to quick before you get the powerband. i wouldnt go much higher than a 70 jet. and some good old fashioned trial and error fun

 

Picture of what ?

 

Oh my bad, here's some photos

 

Rollers are a little heavy maybe. Think I had something like 5gs in mine with a pm360

 

Is it the PM aluminium x-tech 70 kit you have on?

 

I had 5g rollers when it was 50cc, but when I put the 70's kit on it was reving to high.

I'm not to sure what 70 kit I have i paid about £110 for it