I had this problem that I have been chasing for a long time. I just found the root cause last night and thought I would share it with everyone in a step by step "How-To". I do this for a living at the Boeing 777 plant in Everett Wa. Manufacturing Engineer in charge of emergency issues at the body join area. We write the plans to put together the A/P.
PROBLEM: My car runs real rough at startup untill fully warmed up and the exhaust is black at the same time (fuel mixture too rich).
BACKGROUND: I checked the water thermostat. It was OK. Checked the ignition points. They were OK. Replaced sooty spark plugs. Disconnected and reconnected cold start injector, front water temp sensor and rear water temp sensor.
FINAL CLUE: I was reading posts on the Vintage Mercedes Forum concerning this problem and came accross a guy that had the exact same problem with his 1973 electronic fuel injected 450 SEL. His explanation of what was the root cause didn't make sense to me but it led me to the right area (distributer).
DISTRIBUTER BACKGROUND: These older EFI systems are different than newer ones. One of the major differences are how the EFI fires the injectors. These older ones are fired by a 4 pack of points inserted as a module in the side of the distributer under the breaker plate and centrifical advavce mechanisim. Oh, this 4pack of points are not the ignition breaker points.
ROOT CAUSE: Oil from the two cam lobe oil pads (Yes there are two more fule injection cam lobes in the bottom of the distributer housing) or the O-ring seal between the lower section of the distributer and the crankcase oil gets on the lower set of the 4pack points. This causes 4 cylenders fuel injection pulses to be intermitent. Consequently you get rich mixture, backfires and rough running.
PROCEEDURE FOR CLEANING THE 4PACK EFI POINTS:
1. Pull the ditrib. cap and wire it out of the way. Don't disconnect the plug wires to the cap without marking them if you want the cap completely out of the way for easy access.
2. Pull off the brown bakeolite T shaped rotor off and set it on the bench as first part in disassembly line.
3. Pull black ABS dust cover and place on bench next in line.
4. Put rotor back on the dist. and mark where it points to at the rim of the dist. housing. Take rotor off and put it back in line on the bench.
5. There is a Allen wrench bolt at the base of the dist. where the dist. shaft goes into the engine block. This Allen bolt secures a sloted bracket that keeps the dist. in the engine block. Mark where the Allen bolt is within the slot in the hold-down bracket. Extract the Allen bolt with a very long T handle Allen wrench or a 3/8 drive Allen socket on two 6 " extensions (be sure to get the Allen socket out from behind the harmonic balancer if it falls off the extensions.
6. Pull the Dist out of the engine just far enough to undo the 2 screws holding the 4pack in the side of the dist.
7. Unplug the 4Pack and clean. My 4pack had no cover so I just hosed them down with WD40 and a small soft brush. Final cleaning with carburator cleaner. Let dry. Put paper between each set of points and pull it out to final clean point tips.
8. Clean the lower 2 dist. cam lobes and add a little 3in1 oil to the oiler felts at the root of the 4pack.
9. Reassemble in the reverse fashion. Don't forget to reset the ignition points on top of the dist. Be sure you line up all your witness marks or your timing will be off.
My car ran like $%^&* for the last 2 months and now she purs like a kitten. This should be the last that needs to be said about this reoccuring problem.
P.S. This is a post from the SL forum which applies here also.

Note, even if you do mark where the rotor was, you still HAVE to re-time the ignition with a timing light, or your timing WILL be off. Especially if you adjust the breaker points while doing this.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

You work on your Mercedes for a living at the Everett 777 plant? And they PAY you? Wow - my application's in the mail...always did kinda enjoy the Pacific Northwest, anyway, and really like the E-6 "Mercury's" you guys built us for the Navy, to replace our EC-130's...

Just kidding - Thanks for sharing via the post - a very helpful sort of item, indeed.

Stan

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stanbabu@3bubbas.com
59 300d
65 250SE/Cpe
70 250/8
72 250C
78 280CE

Nice wrench!

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe