Fuel injection idle mixture knob stuck - help needed

[Novamonte]

The idle mixture knob at the rear end of the fuel injection pump of my 250 seems to be stuck. I can't press in in and the knob rotates when the engine is running (which I assume it is not supposed to do). Has anyone had the same problem and has any ideas on how to fix it. Can I remove the big hex nut with the knob in the middle? Do I have to pull the pump out of the car?

Clas

[stickandrudderman]

You can just remove the big hex nut and deal with. Simples.

[Novamonte]

Thanks! Any tricks for removing it? It is pretty tight there...

Clas

[wwheeler]

From my recollection, there are a few threads on this site from people who have had this same problem. Use the search function.

I think they found that the mixture adjustment screw was turned out so far that it was engaging the spring loaded adjustment knob. Since the mixture screw spins with the engine, the adjustment knob also spins since it is engaged. This would also explain why you can't push it in.

I have also taken off the hex nut and had to buy a big metric socket. If you do take the nut off, be sure to replace the small o-ring on the adjustment knob.

[Novamonte]

Thank you - this was exactly what had happened. My engine idles a bit rich and in my attempts to cure this I had obviously turned the mixture screw too far CCW. A couple of clockwise turns fixed the problem. A warning signal seems to be that the mixture screw stops "klicking" when the adjustment has passed the maximum.

I had tried the search function on this excellent forum, but I didn't manage to find anything - I must keep honing my searching skills.

[wwheeler]

You shouldn't have to back it off that much too cure an overly rich mixture. It sounds like you have problems elsewhere (there can be a lot of them) and you are compensating with the mixture screw. If everything that effects idle mixture is working and adjusted correctly, there should be more than enough range on the screw to get the mixture correct.

Look at the linkage adustment tour, WRD, Cold start valve (CSV) and possibly the barometric compensator. There are more, but that is a place to start.