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Old 02-19-2010, 02:43 PM   #1 (permalink)
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Air - Fuel Adjustment Screw Location ??

1979 450 SLC

Finding this screw has been driving me crazyyyyy!!! I searched this forum and found information stating that it was an allen screw located underneath the air filter housing, under a rubber cap of some sort.... no such luck...

I attached pictures of what I found...The only screw I could locate has a straight screw head and is right in front of the fuel distributor on the intake manifold.

The car is running way too rich, and I need to adjust the air-fuel ratio so I can test the exhaust for the CO level...and then hopefully get it to the right setting.

Anyone who knows where this screw is please reply....many thanks!!
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Air - Fuel Adjustment Screw Location ??-february-2010-002.jpg   Air - Fuel Adjustment Screw Location ??-february-2010-004.jpg  
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Old 02-19-2010, 04:57 PM   #2 (permalink)
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in middle of second picture, looks like a slotted screw. Mine is allen wrench so it may be under the slotted screw. best to make adjustments in 1/4 turn increments. The underside of my air baffle plate was gunked up, removed it and cleaned with solvent.

Last edited by 1968; 02-19-2010 at 05:01 PM.
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Old 02-19-2010, 09:41 PM   #3 (permalink)
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Thanks 1968! Did not realize that it would literally be underneath another screw. What is the size of the allen key you used?
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Old 02-19-2010, 09:51 PM   #4 (permalink)
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The normal screw takes a 3mm allen or hex inset. I have a feeling that yours has been altered. Try the slotted screw that is there and see if there is any adjustment. Originally there was a plug over the hole and the 3mm screw under it.
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Old 02-19-2010, 10:55 PM   #5 (permalink)
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hmmm - it must have been altered...since there is no cap. But I was able to unscrew the slotted screw, remove it, and then with a flashlight could see the allen screw inside. I will go grab a 3mm allen key that is long enough to make adjustments. And then do 1/4 turn (counter clockwise) adjustments to lean the mixture...

I attached pictures of the screw I removed and of where I see the allen screw down inside. If does not seem right - please inform. Thanks!
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Air - Fuel Adjustment Screw Location ??-february-2010-006.jpg   Air - Fuel Adjustment Screw Location ??-february-2010-011.jpg  
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Old 02-20-2010, 02:24 AM   #6 (permalink)
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The screw you are turning adjusts the sensor plate within the air horn, affecting the A/F mixture at idle only.



The warm up regulator, also called the control pressure regulator is responsible for controlling the A/F ratio at operating rpms and loads above idle.
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Old 02-20-2010, 01:18 PM   #7 (permalink)
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So one can adjust the air/fuel mixture at idle by turning the idle mixture adjustment screw, as seen in diagram above (thanks isthisdave for pasting this)

But how does one adjust the warm up regulator to control the air/fuel ratio at operating rpms?
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Old 02-21-2010, 05:23 AM   #8 (permalink)
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There is a lot of information in the encyclopedia Germanica on the theory of operation of CIS fuel injection which you need to read, it will help you understand the WUR, systems pressures, AFR adjustments, FD etc in some pretty good detail.
It's a sticky at the top of the page.

But if you're running that rich you might want to make sure you have no vacuum leaks before adjusting the AFR.
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Old 09-11-2011, 12:04 AM   #9 (permalink)
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Smile Anyone know the thread size

anyone know the thread size of the 3mm hex screw or a place where i can pick up one? my car does not have one for a strange reason. 1981 380sl
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Old 09-11-2011, 02:27 AM   #10 (permalink)
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When adjusting the 3mm screw do not rev the engine. If you do the metering plate tries to rise up binding your allen wrench.
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