OM621 throttle position guidance
 

Several years ago, a friend of a friend brought me her '62 Heckflosse for a floorpan replacement. I ended up replacing the four pans, most of the unibody frame rails, including custom door cards and modified 124 seats fr/rr.

In the process, I unwisely thought I'd replace her very worn throttle rods with some good ones I still had from an old OM615. Unfortunately, I didn't measure the relative positions between the rods and the throttle body and pump. I have the old paper manuals from my OM615, which state how to adjust and set the throttle rods relative to other items peculiar to that particular engine. This OM621 does not have that same configuration, though the two engines are the same in general principle.

I need some guidance regarding how to reset the throttle so that it will idle without loping and accelerate smoothly without dying. When I initially checked the governor, it wouldn't hold vacuum, so I disassembled, cleaned and greased it up. It now will hold vacuum. It still won't idle. When I got the car, it would run not too great, but much better than it does now.

I desperately need to wrap up this project and return it to an impatient owner, and then of course to regain the space in my tiny little yard. Any help and advice would be greatly appreciated.

Here is an excerpt from the Chilton's manual.

"This [additional mechanical control] mechanism helps to eliminate idle speed variations, i.e., "hunting".

With the idle speed properly adjusted (700-800 rpm), detach the connecting rods and measure their lengths, center to center between the ball sockets."

Connecting rod between venturi and relay lever on valve cover: 310mm
Connecting rod between relay lever and idle "anti-hunting" lever on injection pump: 205mm
Connecting rod between venturi and relay lever on side of engine block: 250mm

"With the rods adjusted, detach the connecting rod [between the relay lever on valve cover and injection pump] from the relay lever and push it down until it rests against the idle stop. In this position, clearance between the ball socket and head should be .04". If it requires more than .04" lift to reattach the connecting rod, unscrew the ball socket."

The connecting rods really only control the "anti-hunting" or loping at idle. Once you open the throttle a little, this lever does nothing. The venturi on the throttle controls the amount of fuel injected into the engine using vacuum acting on the diaphragm in the back of the injection pump (in the governor). The more the throttle is opened, the less vacuum to the governor on the pump, and the more the spring inside pushes the fuel control rod in the pump. If this diaphragm is leaking or there is a leak between the venturi and diaphragm, the pump will inject too much fuel and idle will increase. The diaphragms crack with age and leak, requiring replacement.

Not sure of the problem not accelerating, but suspect it is also in the diaphragm governor. Maybe there is some binding in the governor diaphragm. The spring in the governor can get weaker over time and shims can be added to give the engine more fuel. Below the diaphragm governor on the back of the pump is an adjustment screw with a lock nut, the maximum fuel delivery rate adjustment. This adjusts the maximum amount of fuel injected with the venturi all the way open, or full throttle. Turning out the screw increases fuel, in reduces it. I would not adjust this screw until the diaphragm governor is thoroughly checked out and ruled out as a problem. It should be set correctly from the factory. It is adjusted in until at full throttle down the road exhaust smoke is minimized. If you turn it out too far, you will leave a cloud of black smoke behind you. A little adjustment (1/4 turn) goes a long way. But be warned that the manual says under no circumstance should the engine speed under no-load be allowed to go over 5000 rpm with either the diaphragm spring shims or maximum fuel screw adjustment.

Hope this helps.

Thanks, that's very helpful. I used to have a Chilton's manual for these oldtimers, but I think I sold them at a garage sale.

Your welcome. Don't usually think much of Chilton's or Haynes manuals, but the Chilton's manual for these cars actually has some good info. Also have an old hard cover Glenn's manual that I use to complement the Chilton. Still, nothing beats an original factory shop manual as the Chilton/Haynes are not all-encompassing.