Dr. Pulley 16x13 Sliding Roller Weights for 50cc 4-stroke QMB139 engines . Please select weight from drop-down menu. The following sizes now come in Black Pearl material: 5g, 6g, 6.5g, 8g. Please see photo gallery for an example. Traditionally, gear ratio is changed by the up-and-down rolling movement of the roller weights in the ramp of the Movable Drive Pulley (MDF). Now the innovative Dr. Pulley Sliding Roller Weights slide up-and-down the ramp to change the gear ratio. This unique sliding movement (not rolling) provides exclusive advantages to increase CVT performance. With Dr. Pulley , no abnormal wear will occur even when running in high gear. Please see our Warranty & Returns Policy.