Im currently running a 17.5mm on my minarelli vertical engine 70cc kit with 88 jet and open airfilter now before that i had the same set-up just with a boxed air filter/standard was working fine. Except if i held bike up full throttle for about 1/4 mile at top speed suddenly the revs would drop and engine would cut out,then i would just kickstart and it would be fine I chose to put gear up kit on my bike as even with 70's kit it only did 45mph plus open air filter and raised my exhaust port by about 2.mm(polini sport) I set it all up and everything was fine could hold bike down and do burnouts and really get the wheel spinning.then i thought of the ingenius idea of playing with my fuel mixture screw to try and improve throttle response twisting it early all the way in,bike started but started cutting out with throttle,then bike stopped kicking over so ive put new clean spark plug in and it now starts and ticks over but get no throttle response. Exhaust bangs bike cuts out......ive tried playing with levels of mixture screw no luck please help it only throttles slightly then when revs get high it cuts out....
It sounds like it was running weak and the piston was seizing . removing the filter and box has probably made the situation worse, I think you need to increase the main jet size.
incresae the main jet size and twist the mixture screw all the way in (clockwise) then start it up and just turn the mixture screw 1 and a half turns out (anticlockwise) and that should be about right to be fair haha
