And yes, I can pass air through stem and that hole:

Edited April 29, 20204 yr by BlueZee

I wonder if that orifice equalizes pressure on both sides.

@BlueZee, Thanks for the excellent pics and the video. Another Z mystery solved. Thanks again!!

SurferD, What that orifice does is bleeds the vacuum out of the diaphragm canister that pulls on the throttle after the throttle opener has been actuated. Goes like this...

When vacuum goes really high (like when decelerating with foot completely off the gas), the vacuum control valve will open and allow vacuum to be applied to the servo diaphragm. When that vacuum is applied to the servo diaphragm, it pulls on the throttle linkage and holds the carbs open a tiny bit to limit the intake manifold vacuum.

Once the intake manifold vacuum drops below a certain point (adjustable with a screw), the control valve will close and additional vacuum will not be applied to the servo.

But the cavity inside the servo still holds vacuum, and that little bleed hole's job is to let that vacuum bleed off so the servo diaphragm can relax and stop pulling on the throttle linkage.

another brilliant answer 

so what does the designation on the shaft end mean? I got a backup today and it's marked 71B6 compared to my original 66PH15. Maybe  it's a lot number or date code. Probably one of those things we'll never know.

Edited May 1, 20204 yr by SurferD

Sorry. Can't help you with that one. No idea!

I finished refurb’ing mine. I obviously used paint and not plating. I don’t know what initial pressure setting to use so I adjusted until there wasn’t much resistance. To tighten its counterclockwise. The bellows is under a good amount of compression so it was a little hard to assemble. I upgraded the screws and lock washers to stainless also. As you can see I changed the set screw due it having to drill out the original. Interestingly they changed the bracket configuration on later models and took out the reinforcements. 

Looks great. I remember seeing an adjustment procedure in some of the literature. Probably one of the FSMs, but I don't know what year. I took a quick look and couldn't find it. It's in there somewhere though. Charts, lines, pressure readings... If you can't find anything, let me know and I'll dig again.

Just remember the bellows are an atmospheric pressure compensating device. And the tighter you press the bellows in, the easier it is to actuate the throttle opener. In other words... If you don't have it adjusted in tight enough, it won't do squat. If you have it adjusted in too far, it will hang the idle up too long when you release the throttle.

You want something between squat and too long.

It was bugging me, so I had another look. 1974 FSM Engine Tune-Up section.  ET-16. Thorough description of the system along with way too complicate adjustment procedures.

I stand by my "between squat and too long", but if you want to follow the factory's procedure... Their description is on ET-21 and 22.  Good luck. 

 

good find I have a 1971 FSM but didn't verify if it has the procedure

Edited May 7, 20204 yr by SurferD

I don't have a copy of the 71, but I found some adjustment info in the 72 manual. EC-19 and EC-20.

Question to @Captain Obvious and @SurferD: are your servo diaphragms (item 5 on a drawing) airtight? I am asking, cause I've checked two that I have in my garage - and both are leaking.