This is a known problem on NiWumpf and also to a slightly lesser extent on Pascal boards. Not much you can do about it. On some of the older games there is a second tilt switch on the playfield. This is not the slam tilt. It is wired to the same spot in the matrix and it can add some extra wire and resistance to the circuit making tilt less responsive.

You can set the switch on the playfield really tight and it will make up for the plumb bob a little bit. Problem is that switch is a little prone to tilting through to the next ball or player.

In my Joker Poker with Pascal all in one board, I ran a parallel wire up to the CPU and it didn't help. You could move the machine to where you could hear the tilt banging back and forth and nothing. I tried everything including an offboard switch relay circuit. I finally got it working by installing an electronic tilt board. It's a prototype and as far as I know they are not for sale yet. It does work great, I have the plumb bob disconnected and calibrate the tilt now with an app on my phone.

I remember seeing a Genie with Niwumpf in the Finals of a tournament. It was surrealistic. All four players were dancing with that machine and it would not tilt.

I spoke with the lead tech last year when I was prepping a Sinbad for a big tournament. They are definitely aware of the problem. The best solution at this point is rubber footies, and tight leg bolts to ensure the game is basically bolted to the floor.

There are some signals that go through both J6 and J7 if you look at the original manual. This probably helped keep the harnessing simple or something. Either way they all end up at the same place on the board.
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A capacitor helps make a marginal switch that is intermittent a little more reliable by stretching out the pulse. In this case the switch contact is simply too fast. A capacitor might help a little bit, but I didn't see it in my test. I suppose it might make a game tilt twice out of a thousand hits instead of once haha.

Before I got the electronic tilt in my Joker Poker, I was seriously considering adapting a relay circuit from an EM into the game. I'm also going to remind people that I have a Pascal board in this game and it doesn't work at all with the plumb bob. I've tried a Niwumpf in this game and it doesn't work either.

Great job guys!!! I sold my Sinbad to a good friend. I'll put together a kit for him. My Joker Poker is working fine with the electronic tilt, but if I ever sell it I'll put the plumb bob back in and keep the electronic tilt.

I really appreciate the effort made by JethroP and Quench. I went ahead and disconnected and removed the prototype electronic tilt board from my Joker Poker and reinstalled the good old plumb bob. I used a nice new plumb bob with thumb screw from PBL along with the new diode and capacitor. The tilt now works perfectly I am glad to report. Now if only the foam ear plug trick would work on these old Gottlieb games without replacing the plumb bob hanger and shaft...

I like the original tilt mech better than the electronic one for a number of reasons. The electronic one worked just ok, but the software was in a very early stage and needed a lot of work to be a really good replacement for the plum bob. Being a beta tester, I wrote up a long list of things that I would change and sent them to the person who is designing the board. Unfortunately I am not sure he is still working on this project as I have received no response.

Edit: Here are the parts I ordered. Diodes and capacitors are literally a dime a dozen, so I figured I would order cheap assortments that had the proper values in them. This will give me enough to do a bunch of games and who knows when I will need some other diode or cap for an old Bally game or something? Anyway I found a diode assortment for $5 shipped and caps for $10.

https://smile.amazon.com/dp/B01M7S2ROI/ref=cm_sw_em_r_mt_dp_U_CfAhFb0S67GFP

https://smile.amazon.com/dp/B085RDTCCV/ref=cm_sw_em_r_mt_dp_U_VjAhFbBCCTVF5