I got my 2008 X18 from MPR, cut the lower body work off. The easiest way is to split the body at the bottom and lift the whole body off. My first time was a nightmare, kept burning my hand trying to work on it, so I cut the windsheild down, and cut the lower body off and got a HP X12 side exhaust and made it fit. See photos.
The newer bikes might have a stop inside the throttle housing. The new carbs don't have an air/idle screw. You might need to make a bigger pilot jet & main jet size. If you have the money, you can change the PZ19mm carb and get a 22mm carb on eBay for $25. The carb may have an adjustable main jet needle on the carb piston. Moving the clip down a slot makes it richer at idle and vise versa.
It should go faster than 40mph, my modded one goes over 60mph.
The newer bikes might have a stop inside the throttle housing. The new carbs don't have an air/idle screw. You might need to make a bigger pilot jet & main jet size. If you have the money, you can change the PZ19mm carb and get a 22mm carb on eBay for $25. The carb may have an adjustable main jet needle on the carb piston. Moving the clip down a slot makes it richer at idle and vise versa.
It should go faster than 40mph, my modded one goes over 60mph.