From time to time we see posts about the ball not getting kicked out to the shooter lane reliably on a Gottlieb EM. Either it takes several attempts, or if it does make it, it does so slowly.
There can be several causes involved, but I thought I'd share some detail on one cause that may not be so obvious.
Here's a standard ball return kicker:
Looks pretty decent, yes? It's clean and shiny, the fork is straight and isn't burred/mushroomed, and the plunger and linkage are tight. It even has a new spring on it (and the correct one).
But with this kicker, the ball would not make it to the shooter reliably (if at all). So some quick checks: the coil was correct (A-1496) and working great - new sleeve and all. Kicker isn't gunked up, and the spring is good. Score motor contact at 4C was clean and making good contact (as was the contact on the 'O' relay).
So why wouldn't this apparently good kicker get the job done?
Well, the way these things work is that when the coil activates, it pulls in the plunger with a rapid stroke, actually fast enough for the spring to stretch and store some energy. This 'shock absorbing' effect then takes that stored energy and moves the fork to launch the ball forward. The key here is that it launch the ball forward, and up into the ball trough ramp. If it's too weak, or shoots up, it either can't make it up the ramp, or (more likely), the ball is knocking on the underside of the apron and losing momentum.
A quick check is to lay your hand on the apron when the ball kicks out. If you can feel a thunk from the ball, it's hitting the apron. If you take the apron off, you'll see a skid mark from where the ball has been hitting it. (And actually, if you take the apron off, then the ball launch may actually start to work a bit.)
So what's going on then?
Taking the kicker apart further reveals the problem. At some point in this game's life, the original spring probably broke, and an incorrect substitute was used. And the kicker just got a whole lot of mileage on it (this particular game has at least 150K plays on it). Despite all that, it still sorta kinda worked, but not very well.
Running the kicker for that long under those conditions takes a toll on the post and the holes. Note how the post is worn down on two notches, and the post hole is enlarged.
When this happens, there's too much play between the sliding plates, and the energy is lost, such that the spring can't provide enough force. In this case, the ball was launching up into the apron underside, and then knocking back down onto the trough.
Unfortunately, there's not a whole lot to do about this but replace it with a good one. But in doing so, make sure the correct spring is used (the back of the '78 parts catalog will indicate which spring is correct for the games covered there). The wrong spring will result in the incorrect force too.
So the next time a game exhibits the weak ball return kick behavior, here's something else to check.