I have adjusted the cable to knob on dash, and the obvious adjustment screws on the throttle body. What seems to be keeping the idle fast is a rod/linkage that goes from the throttle body straight down the side of engine to a lever coming out of (? I don't know what).

What does this linkage do? Can I safely shorten it - which it looks like would solve my idle problem?

Could you take a picture of which rod you are referring to?

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k

I had a idle problem with my 73 that turned out to be a ruptured diaphragm in the injector pump. IF yours is the same as mine.. you will have a vac hose that attaches to the air horn right by the throttle blade and goes down to the back side of the injector pump. With motor running slowly place your finger over the vac hose inlet in the throttle and see if she slows down. If it does this diaphragm may be leaking.
If it is leaking it can also suck the oil out of your pump. So you may want to check the oil level as well. It's a real PITA to install and costs about $90

Hope this helps

__________________
"Any sufficiently advanced technology is indistinguishable from magic"
-Arthur C. Clarke

"I won't be wronged. I won't be insulted. I won't be laid a hand on. I don't do these things to other people. I require the same from them."
--John Wayne.

OBK member#16
77 300D
75 300D (rusty parts car)
73 220D (Dad's)
98 Jeep Grand Cherokee
82 Kawasaki M1

Could be idle fuel mix. The idle fuel mixture adjust is the 14mm collar nut with phillips screw inside of it that goes into the injector pump topside close to the engine block. You loosen the collar then turn the phillips screw. Best method is use a 14mm deep socket turned with visegrips and screwdriver inside of it, then you can tighten the collar nut keeping the phillips screw set where you want it.

If it was me I would not be messing with lengths of linkage rods.

Thanks for your input guys. I am about to attempt to post some pictures as Diametricalbenz suggested.
There is a vacuum line from throttle body to injector pump but I couldn't get it off at either end to check for vaccum leak. I don't see anything that looks like the idle fuel mixture adjustment screw described by 240Dog.
There are linkage rods from the throttle to the injector pump such that the throttle blade and a lever on the side of the pump always move together. This is shown in my pictures ( if I ever get them posted) as rods 1,2&3.

Sooo... I'm not sure what to try now. Any suggestions will be appreciated.

Removal of the vac hose is not needed. Just gain access to the air horn, and you will see where the hose banjo-bolts to the underside of it. Then just slide your finger over it's inlet right by the throttle blade.
Mine is a 73.. so I'm not sure if yours is the same. But it sounds close. Sure you will figure it out.

__________________
"Any sufficiently advanced technology is indistinguishable from magic"
-Arthur C. Clarke

"I won't be wronged. I won't be insulted. I won't be laid a hand on. I don't do these things to other people. I require the same from them."
--John Wayne.

OBK member#16
77 300D
75 300D (rusty parts car)
73 220D (Dad's)
98 Jeep Grand Cherokee
82 Kawasaki M1

Oldblue, your best bet may be a decent manual. The CD version sold here is a good investment if you plan to keep the car.

In layman's terms, the rod assembly that goes over the valve cover and to the side of the injection pump is called the tickler. Adjusts the injection pump rack at low engine speeds to keep the engine from loping - although "panting like a dog" may be a better description. At low engine speeds there isn't enough airspeed in the throttle body to create vacuum for the governor.

If it was me, then I'd;
1. If you have a "gorilla knob" glow/start switch then make sure the pin floats in the cable end slot when in the run position.
2. Disconnect or loosen the hand throttle cable so it has no effect.
3. Clean and lube the ball joints.
4. Make sure the firewall rotating rod is rotating freely. If you see cracks in the bushings then it's definitely time to replace. This is the most common culprit.
5. Pull the air ducting upstream of the throttle body and look inside the throttle body. Shouldn't be any gunk. Clean and lube the butterfly valve shaft ends on the outside of the throttle body.

Then (and only then) I would consider making an adjustment, and only with the manual at my side. An easy procedure providing you follow the steps in order.

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daBenz - 1970 220D