I have a 250SL that is running so rich that it is not driveable. The cold start valve has been replaced, new plugs and points have been installed.
The problem seemed to begin immediately after having a new water pump installed although I expect that this is just a coincidence.
Any suggestions for what I should check?

 

carburetor?

 

This car has mechanical fuel injection......no carburetor

 

I assume the electrical circuit that operates the cold start valve was checked for proper operation. I think it actuates the cold start solenoid for 10 seconds on a cold start. Also, there is a thermal bulb on the FIP (has two hoses connected to it) that acts to close off the auxiliary air valve as the coolant temp rises. That is why there is the little air filter on the FIP. I can't remember for sure, but I think that thermal bulb actually richens the mixture during warmup (by acting on the governor).

If that is OK, then you probably need to have the FI Pump serviced. The fuel rack or the governor may be sticking. There is a way to check if the rack is sticking in the car but you really need to understand the pump internals and operation before tinkering.

Also, you should join this site: www.sl113.org

I had a 250SL for 36 years, but I sold it 9 years ago so I am working from memory, so take my advice with a grain of salt.

 

Thanks Vince
The cold start valve and the electrical controlling it has been checked and is working properly.
I am going to have the mechanic check the thermal bulb that you indicated. He is on vacation for 2 weeks so I may not reply until then but I do appreciate your comments and interest.