Odd CO% mixture question

[Michael C]

Okay, here goes, I have a 1970 W108 with an M130 motor (Same as my 280SL).  Here is the question.  On the 1970 model year the CSS is only used for the A/C.  While in gear (Automatic) a vacuum pot is used to open the intake a little allowing more air in thus raising the idle.  The only issue I can see with this is that when you put the car in drive, it leans the engine while sitting at a stop.  If I set the CO% at the suggested 3.5-4.5% while in park, it lowers to about 2% in drive at a stop.

Should I be adjusting my mixture while in gear?  (My SL of the same year has a 4 speed and don't have these issues)
Gotta love these early emissions

Thanks

Michael

[Benz Dr.]

That's an interesting question.
 I would say that you can leave your idle mixture ratio where it is. When the throttle opens the engine RPM should remain about the same because it's under load. There's more than enough fuel coming through the injectors and all you've done is add a bit more air when the throttle opens.

 You should be OK at 2% when the throttle opens but it wouldn't run at all if you set the idle mixture that way. The engine would lean out and stall at a 2% setting but not at 3% or higher.

I wouldn't worry too much on the numbers and I'd concentrate more on getting it to run smoothly in gear.   

[stickandrudderman]

Firstly, i have never seen such an arrangement on a 130 engine. I have seen what looks like a vacuum pot but is in fact a pressure cushion that slows down a closing throttle. Can you post a picture?
Secondly, a vacuum pot does not introduce any extra air unless it's failed. It simply utilises the vacuum present in the intake manifold to do mechanical work.
A good example of this is the much discussed (recently) brake servo (booster). The application of the brakes does not change your CO unless the booster has failed.

[Michael C]

Not sure how to post a picture on here but if you look in the BBB page "07-14/7"  and look at figures 07-14/21 and 14/22 you a picture & drawing.

[ctaylor738]

You would think that if the vacuum pot moved the linkage, it would move the lever on the pump and you would get a little more fuel to go with the air.  Maybe there is some slop in the linkage that is preventing this. 

[GGR]

CO goes down with rev. I f you happen to driveyour car with an on-board measuring device you will note that the CO is around 4% at idle and then will go lower when you drive. So I imagine that by accelerating idle for AC you just see what happens during normal driving.

[Michael C]

Thanks ctaylor.  No slop on my linkage.  On the 70 & 71 Mercedes used a "Slip linkage" on the rod going to the butterfly.  This allows the vacuum pot to open the intake butterfly without moving the rest of the linkage.  It's a pretty strange set up.  At the time of design, it was a good way to lower emissions at idle while in gear.

[ctaylor738]

Well, I'd say it's operating as designed.  Seems odd that in gear and under load when a slightly richer mixture would be desirable, the mixture by design would go leaner.  If in-gear idle is a problem, you can always fatten the mixture a bit.

[jacovdw]

...I would say that you can leave your idle mixture ratio where it is. When the throttle opens the engine RPM should remain about the same because it's under load. There's more than enough fuel coming through the injectors and all you've done is add a bit more air when the throttle opens.

 You should be OK at 2% when the throttle opens but it wouldn't run at all if you set the idle mixture that way. The engine would lean out and stall at a 2% setting but not at 3% or higher.

I wouldn't worry too much on the numbers and I'd concentrate more on getting it to run smoothly in gear.   

I agree with Dan.

This is exactly the same setup that is found on the W114's with the Zenith carburetors.
If the engine is under load at idle and the speed drops, the vacuum in the manifold drops as well.
The resulting drop in vacuum causes the spring loaded plunger to extend and hence increasing the idle speed slightly to prevent stalling.

The main aim is to adjust the engine idle speed with the vacuum regulator to prevent the idle speed from dropping too much when, for example, you do the following:

a) switch the AC on
b) engage Drive whilst stationary
c) turn the steering wheel whilst stationary (cars with power steering)

If you adjust the mixture with the transmission in D, you might lean it out too much and stall/stumble.

Attached, is a picture of what Michael is referring to.