Warm-up Regulator "Tweak"
 

I have been told that there is a way to "tweak" the warm-up regulator. I am having a problem with rough idleing when cold.

Way back in the good old days we used to dis-assemble them and drive a certain pin back out a little. It's been so long since I"ve done it though, I can't describe it from memory. If I saw one I'm sure I'd remember. Anyway, after re-assembling and re-installing it I would connect a fuel pressure guage and let it sit overnight. IMMEDIATELY following cold-start, I would tap that pin (previously driven out ) back in a bit until the fuel pressure was back in spec. This cured a severe hesitation on acceleration when cold. If I remember correctly, this procedure is not found in any repair manual or MB service literature. It was just an inexpensive alternative to replacing the regulator. I hope this helps

You probably don't need to adjust it as much as you need to clean it. There is a screen in the intake side of the regulator that gets clogged which raises the control pressure in the system which in turn makes the engine run leaner than it should during warmup.
I would remove the regulator and take it apart(carefully, make notes if you need them) After you remove the 6 screws from the cover and the clip at the bottom of the electrical connection flip it over without separating the halves, remove the top and note the position of the little pin and mexican sombrero looking piece(the pin is under the sombrero just keep in mind that it is there) remove the 10mm nut and the flat spring(bi-metal) , remove the sombrero and the pin which is underneath it then unscrew the little silver disc down in the bottom, clean out all the passages you see with a spray can of carb cleaner and make sure you get the cleaner to wash back through the fuel line fittings which will clean the screen. reassemble it and bolt it back on. I may have missed an additional spring in there but it will be self explanitory.
There is really little chance of messing it up so tinker with confidence!

Thanks for the help but I think the car is running RICH, not lean during warmup. I know this because there is a BIG black streak under my tailpipe on the garage floor and black smoke greets the folks on the freeway when I pull out in front of them (come on, when you drive a big SEL, they expect that kind of thing).

In that case you might be better off searching for another regulator. You can adjust it but it is a little tough to explain how to do. In short you have to dissasemble the regulator and punch the 1/4" round plug you see in the top of the regulator back out until it protrudes about 1/8" above the casting. Then you reasemble the reg connect a fuel pressure gauge and with a cold engine and the electrical connector disconnected you pound the plug down until the proper pressure is achieved. beating it in richens the engine so it has to be pounded up from the inside to lean it out.