pop bumpers that stick down seem to be an issue with my Surf Champ machine. I'm pretty sure I identified the left one as the culprit. After cleaning out the coil core and inner tube, I first tried putting a larger spring in there (cut up a launch plunger spring and enlarged it).
That did the trick, but the coil having a harder time pulling down, and not getting all the way down, makes the ball shoot off slower and not hitting the drop targets hard enough.
Now I think the metal rods also rub up a little against the playfield holes. I was wondering if there would be any downside to drilling them a little larger, since they're hidden by the plastic disc anyway.