I have found sometimes that this can happen when a combination of the rotor, lifter & throttle cable are worn, as the rotor turns, the top edge of it lifts & lowers the lifter (the 3 arms of the lifter rest on the tracks at the top of the rotor) with age, these tracks, and the slight chamfer on the end of the lifter arms can become un-even, and jam-up, this becomes worse as the throttle cable weakens with age & minor kinks. it can also be furthered by use of the wrong springs.
It can be fixed by tidying up the lifter arms, and rotor tracks, fitting new springs and/or throttle cable, or by replacing the lifter and rotor.
