Jerky upshifts or downshifts?

I thought the modulator always vents a limited amount by design?

The FSM and the Internet seem to be silent on that point. Anybody got a NEW modulator they can test to find out if it pumps up vacuum or not?

Given that I have 15" of vacuum at the VCV but drops to 9-10" with the modulator attached, that says a pretty decent vacuum leak to me.

Rough 1-2 upshift, jerky downshifts under load. Coasting downshifts aren't objectionable.

With the modulator under vacuum, 1-2 is smooth, 2-3 flares, 3-4 varies. Downshifts under load are better, not perfect, but better than with no or little vacuum.

I seem to recall replacing an o ring in the modulator which corrected the amount of venting. With the vacuum losses you are seeing perhaps that may save you from purchasing a new modulator but the new ones are considered an upgrade.

An update and to put this thread to bed. Long story short, I got it figured out.

I forgot that I had diagnosed the BFS as being bad previously and ordered a new (used) one early last year when I started working on the car. Was going through the trunk this afternoon looking for a tool (all my parts and tools are in the trunk for the moment) and found it sitting there so I installed it.

Additionally I checked the VCV. The shaft felt fairly crunchy when turned and it would go through points where the ports were open and others where they were closed. I had a spare VCV off the original IP and tried it instead. As the shaft is turned on it, it gets to a point and closes off vacuum. Seems more like it should be, so that VCV got installed and adjusted per FSM procedure.

Reconnected all the vacuum lines where they're supposed to be and verified that both switchover valves were operating and open (they were). Crawled under the car and checked the rubber nipple on the modulator, which was a bit crusty and changed that out.

Went for a drive, and it was like a completely different car. Seriously one of those "you have to witness it for yourself to believe it" kind of things. Upshifts are good and happening at the right times, and downshifts are basically unnoticeable. Acceleration downshifts are fairly smooth (something this car has NEVER done). I have the Bowden cable set about where I want it too. At WOT without pressing the downshift button, upshifts happen ~4400 RPM.

2-3 is still a little sloppy, so I think I will replace the K1 accumulator spring when I have the pan off, but as it is, nothing else seems to need to be changed. I still wonder about that modulator, but after replacing the rubber nipple, I'm able to pump it up to ~17" Hg, so I'm leaving it be for now.

MANY MANY THANKS to those who offered their expertise for this task! It's a hideously complicated system and with 30 year old parts that aren't working like they should, it makes it damned difficult to troubleshoot. Who knew something as seemingly minor as vacuum adjustment could affect the transmission shifting that profoundly?

I simply can't believe the difference in how the car drives now. So quiet and smooth in town. Timed the 0-60 speed this afternoon and I'm right at 12s, previously was ~13-13.5s