1978 450 Sl - PeachParts Mercedes-Benz Forum

Tims450SL · #1 03-20-2008, 09:30 PM

My new SL seems to stutter then catch and go as in runnning great. It also pops from under the hood on occassion. What is that? A fuel filter issue? Thank you!

JimFreeh · #2 03-21-2008, 04:44 PM

Prolly need to tweek the CIS.

There's an enrichment adjustment you can set with an allen wrench.

Of course, it could also be a clogged air filter, bad injectors, bad fuel distributer, vacuum leaks, etc.

Jim

Tims450SL · #3 03-21-2008, 11:43 PM

where is it located? All other things are replaced. CIS Location??

Tims450SL · #4 03-21-2008, 11:43 PM

also how do I know it is ok when I adjust it? Any tell tale ways to know when it is ok by adjusting it?

JimFreeh · #5 03-22-2008, 09:47 AM

There's a port in the air cleaner that allows you to access the adjustment.
It's a cap screw that uses an allen wrench. Located next to the plate in the intake.

An O2 sensor in the tailpipe helps adjust the mixture.

Often, an older CIS car needs to be tweeked a bit richer for better running.

Tim, given your mech level, have your mechanic check this out, it's a sensitive adjustment.

Jim

Robert Squires · #6 03-23-2008, 06:49 PM

* Tim, the "accelerator pump" function in the CIS-K system is handled by the control pressure regulator, also called a warm-up regulator. That is to say that the control pressure regulator takes care of that extra bit of fuel you need when accelerating, just like the accelerator pump circuit in a carbureter.
* You'll need to hook up a fuel pressure gauge and test the control pressure, both with and without vacuum to the lower chamber of the CPR. If there's no change in control pressure when you remove vacuum (engine idling) to the lower chamber, then you've found the cause for your acceleration flat spot.
* As you probably know, fuel adjustments are done dead last. When you're setting up an engine, check everything else first. Cam timing, ignition advance/retard, etc. I'm assuming you've got good compression and valve action.
* Set your idle mix, when everything else is dialed in, to "lean-best idle" ( a useful term from the old carbureter days). In the absense of an exhaust analyzer (pre lambda) or a duty cycle meter (post lambda), using a tach to adjust to lean-best idle is pretty close. I don't adhere to the "richer is better" philosophy, even for a 4.5. Set it as close to factory specs as you can.