I've been struggling with bad ball trough feed for a few years.
That is when the coil should release a ball to feed the upkicker, the balls remained stuck, and not advancing.
Adjusted the switches to the most sensitive way, replaced all 13 (!) balls to non magnetic, cleaned the full mech, checked and fixed any pits in the trough rail, checked game pitch and leveling, etc... no solution
I can now say I finally fixed the problem by doing this.
I added the extra rail to the trough that was later added to sega/stern machines. For example IJ4 had a seperate update kit for this that had these parts:
trough ball guide plate 535-7801-00 (1 pce)
1/4" x 5/16" x 0.144" I.D. Spacer Tap. 254-5014-03 (1 pce)
#2-56 x 1/2" HWH-SER TF (Larger Head) 237-5937-01 OR 237-5937-01 (6 pcs)
In case anyone has thee same problem, this is your fix!
download (resized).jpg