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Runs rough after a few minutes

I have a 1986 Mercedes 560SL convertible. It runs fine when I first start it up but then, after a few minutes of driving, it starts running a little rough like it’s missing on one cylinder. Sometimes I hear a little pop back through the carburetor just before it starts running rough. I changed the plug wires but that had no effect. Any suggestions as to what might be causing this would be much appreciated.

This car should use a CIS based fuel injection system and these systems can be very contrary.
The first step is running the fuel pressure tests and this requires a special pressure tester. The generic ones sold at auto parts houses will not work.
When I priced this tester years ago it was about 400 bucks so I made my own.
If you do your own service work on a CIS car you really need to have a pressure tester.

So you will need the pressure tester and the troubleshooting manual for CIS before you could even start on this problem.
Problem in the control pressure, the fuel distributor, etc.; who knows.

I will point out that CIS is extremely fussy when it comes to air leaks so make sure there are no air leaks. That’s probably not much help but it’s all I got from my vantage point. My SAABs use CIS and it’s wonderful when it works; a royal PITA when it’s not.

Thanks. I’ll pursue this and see where it leads. BTW, I love SAABs. Owned two 900s years ago and haven’t found anything since that I enjoy driving as much. I’ve had my eye on a 95 or a 93 convertible but haven’t gotten serious yet.

At least the newer SAABs have a “normal” fuel injection system and the CIS quirks are a thing of the past.

Yes, they are fun to drive. I love a manual transmission and when coupled with a turbocharger the excitement level goes up a bit. :slight_smile: