Barometric Compensator

[Harry]

1966 230SL, Automatic.

Just curious about the construction of the Barometric Compensator (BC).  I don't find a complete sketch in the Manual or on the Forum although the latter does provide several photos that suggest its construction.  So here are my thoughts based on what I read and see.  Within the outer can, there is a sealed bellows that is sealed against the top of the can.  There is a spring within that bellows (that provides additional force to supplement movements of the bellows).  Below the bellows and within the can, there is a pin that is supported by a lighter spring that simply holds the pin in the upmost position (against the bellows).

This pin protrudes below the bottom of the can and acts upon the fuel injection pump rack adjustment to adjust mixture.  As outside atmospheric pressure drops (as indicative of higher altitude and less air), the bellows expands pushing the pin into the rack adjustment.  This results with the FIP providing a leaner mixture to compensate for the thinner air at altitude.  But where is the opening in the can for atmospheric pressure to enter the non-bellows area?

I removed my BC and it appears that top of the pin is measures about 21.33 mm from the base of the hex nut, so it appears to be operating properly - although testing needs to confirm this.  But it doesn't appear to be "over-extended" which would suggest a leaking bellows.  I have just felt that the car is running a bit rich.  The gas mileage is around 17 on the road and I think it should be closer to 20.  Plus the car smells a lot like raw fuel when first started.  Maybe it's in this old head?

Thanks,
Harry

[Harry]

Photo attached.

[Harry]

Also - the BC has a single aluminum spacer.  22.5 mm (0.085 ins) thick.

[BobH]

Hello Harry, did you see this?

https://www.sl113.org/wiki/Engine/AltitudeCompensator

https://www.sl113.org/forums/index.php?topic=11419.0

[Harry]

Hello BobH.

I did find those posts and they were helpful but didn't quite give enough information to give me an image of how the BC is actually completely assembled.  More curiosity than anything else.  It certainly looks as though mine is functional but I will probably test it in the "bell jar" configuration.

I wonder what the "typical amount of shims should be, realizing that these will vary?  My BC had a 21.6 mm single aluminum shim beneath it.  This is almost an order of magnitude greater than jeffc280sl reported under his, and would result with a much richer mixture - what is what I think I'm seeing.  I could simply re-shim to the order of what he found and see how the results appear.  I just don't want to result in a position that has the engine running too lean.

I don't have a means of measuring exhaust mixture but I could certainly get one, I suppose.  I'd like to have some feedback on which is the best for these cars.

Thanks,
Harry

[rwmastel]

1966 230SL, Automatic.

The gas mileage is around 17 on the road and I think it should be closer to 20.

By "on the road" do you mean mixed urban & highway or strictly highway?  You think 20mpg because... ?

Also - the BC has a single aluminum spacer.  22.5 mm (0.085 ins) thick.

You mean 0.85 ins?

When you test it in a jar, share some pictures.  I'd like to see how that works.  Is there a chart that shows post projection length compared to pressure?  How would you measure it while in the jar?

[Harry]

I thought the manual called for that mpg on the highway.  Plus, the plugs are a bit sooty and there is a prevalent smell of raw fuel.  Runs good on the highway just seems a bit off.  Plus my shims sound like they’re well beyond what I have read.

Thoughts?

[kampala]

Also - the BC has a single aluminum spacer.  22.5 mm (0.085 ins) thick.

Can’t help with your issue but you say your spacer is 22.5mm - do you mean that this spacer is your shim?   If so, something’s is really different here as 22.5mm is almost an inch thick. 

[Harry]

Kampala,

Thank you for pointing out my error!  The correct conversion for the 0.085 ins is instead 2.13 mm.  It’s a classic example where one makes an error in the math and doesn’t stop for a minute to make a “sanity check”.  😳.

Harry