You can modify 139QMBs, the engine and transmission components of many scooters, to improve your scooter's speed. Prior to modifying a 139QMB, you must recognise two different types of scooter speed: acceleration (pickup) and cruising (top-end). 139QMB engines provide a balance of these two speed types, though some driving scenarios may not require as much of one type of speed as the other. For example, rolling hills with lots of traffic lights and turns require more acceleration speed than cruising speed. Making your 139QMB scooter go faster requires you to choose what type of speed you want.
Determine the type of speed you would like your 139QMB to have more of. With its factory-installed, continuously variating transmission (CVT) pulley wheels, the scooter has an optimal balance between acceleration and top-end speed. If you want your scooter to have more pickup, you will install smaller CVT pulley wheels. If you want your scooter to have a higher top-end speed, you will install larger CVT pulley wheels. Regardless of what type of aftermarket pulley wheels you install, you will be sacrificing one kind of speed to enhance the other.
Loosen the head of the pin holding the kickstart to the CVT case with a socket wrench. The CVT case is the long, cylindrical case on the left side of the scooter. The kickstart is the peg that sticks out of this case that you press down on with your foot in order to manually start your scooter.
Unscrew the six bolts holding the lid of the CVT case in place with a socket wrench. This will open up the CVT case, giving you access to the pulley wheels in the CVT system.
Remove the larger, rear pulley wheel in your 139QMB's CVT system. The rear wheel will have smooth edges, so you can either remove it with an air-powered impact wrench or a strap-and-socket wrench combo. If you're using the combo, wrap the strap wrench around the outside of the rear pulley wheel and tighten it so you have leverage against the wheel's natural inclination to turn when you attempt to unscrew the bolt holding it in place.
Purchase a CVT pulley wheel that is larger or smaller than your 139QMB's rear wheel, depending upon your desired speed modification. You can buy CVT components through online or physical catalogues or at your local motorcycle dealer. Use the wheel you removed from the CVT for guidance as you shop for your replacement wheel.
Position the replacement wheel where the former wheel was once located.
Insert and hand screw the bolt holding the replacement wheel in place.
Finish tightening the bolt holding the replacement wheel in place using an impact wrench or a strap-and-socket wrench combo. Again, wrap the strap wrench around the outside of the replacement wheel, tighten it, and hold the handle in place as you tighten the bolt with a socket wrench.
Rescrew the bolts holding on the CVT case's faceplate.
Pin the kickstart back in place and tighten the bolt head of the pin.