When playing a Bally Eight Ball at a local arcade, I was helped to large scores by the left slingshot not working; there was no left return lane, so the flipper was right next to the lower corner of the s/shot. With it not working, it was easy to trap the ball if desired, and to hit the important 'Bank Shot' from the left flipper, advancing bonus multiplier
As to adjusting slingshots, it's a bit vaguer than for bumpers. One certain point is to use the right size band, and stretch it/them evenly*. The adjustment of the longer blades, which touch the band, controls their 'travel' as the solenoid kicks, and the return imparted by the band. Adjusting the shorter blades, even by bending the bracket, affects the gap. With older Williams games (and possibly post merger Bally ones) using the slings as 'Special' solenoids, a switch remaining closed^ will lock-on the coil. Williams had stopped using this method by the mid System 11 games - 11B games like Cyclone, with 50v slingshot coils, won't have it. Too close a gap allows the longer blade to close the contacts again as the band rebounds, causing 'chatter'
* I was called back to a Whirlwind I'd just serviced because the r/h sling was kicking randomly and cancelling the Skill Shot due to this
^ Even a ball jumping inside the rubber band and touching the switch can do this. Old Gottliebs often have headless nails behind the band to prevent access
