When I first got this car I had a terrible time with massive flaring. 1st to 2nd, 2nd to 3rd....
You get the picture.
I checked my vacuum and had almost 22 at the main valve and it only dropped to about 8 when the bleed valve was totally open. To fix this the first time for me was I dialed the modulator way up and I mean like 1.7 turns (seems that if you go past a certain point you return to your starting point, maybe just a 360 degree adjustment) to get the shifting to harden up.
Problem was that in order to eliminate the flaring it was too hard in all shifts and late in upshifts with only partial pedal.

After reading all the information that I could find I thought I would try to shorten the range of vacuum and then readjst the modulator. First things first though...

It seems that my vacuum bleed valve on the IP was out of adjustment. It was not opening fully at full throttle so this was the first adjustment. Set full throttle and adjusted the shaft to within .005 of the stop on the bleed valve. This caused slightly better shifts with the modulator backed off one step but the upshifts were still pretty harsh in all gears. Vacuum dropped to about 7.6 or so at full throttle.
I then went to the local auto parts store and found a vacuum connector with a very small hole in it. I placed this between the main feed line and the vacuum switchover valve on the valve cover (btw - mine has the simple three way that controls only tranny and egr, not the newer one with about 6 connections...) this had the effect of reducing my at idle vacuum at the valve to about 15 from 21. Now with the bleed valve correctly adjusted at full throttle it drops down to about 6.5 at full throttle instead of 8.
I then turned my modulator back about 2 notches and low and behold my shifts are pretty darn good. Slightly hard from 1st to 2nd, very mild flare (smooth) from 2nd to 3rd and clean from 3rd to 4th.

I post this long winded message for any of those like me who need ideas on how to fix their vacuum controlled shifting and have no bowden cable to solve their woes. I thought about using this TransX product to tighten up my seals and gaskets in the tranny but am not one to add aftermarket products too quickly. This might help too.

This whole system makes me really appreciate the simplicity of the simple single rod connecion on my 115 but it appears that I finally have this one solved.... for the moment anyway!

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

Good post. Thanks Fisherman!