OB experts - idle adjustment screw
05-15-2006, 10:46 AM
#1
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OB experts - idle adjustment screw
What does this screw actually do? Yes I know if you turn it, idle goes up or down. What exactly is it doing though?
Is it presetting the throttle just a tad to allow air by? Is it somehow leaking air past the throttle?
Call me curious.
Is it presetting the throttle just a tad to allow air by? Is it somehow leaking air past the throttle?
Call me curious.
05-15-2006, 10:54 AM
#2
Burning Brakes
By adjusting the screw, the air sensor linkage geometry is essentially changed. It does act as somewhat of an idle fuel/air pre-set, but it primarily changes the amount the fuel distributor piston is moved relative to the air sensor movement.
05-15-2006, 10:55 AM
#3
Race Car
Your second assumption is right on. It leaks air past the plate.
Which gives me occasional fits because you can get different AFR settings to work at idle, that are then less than optimum throughout the rest of the curve.
Which gives me occasional fits because you can get different AFR settings to work at idle, that are then less than optimum throughout the rest of the curve.
05-15-2006, 10:57 AM
#4
Burning Brakes
Sorry, I was describing the wrong screw (CO adjustment).
By adjusting the idle screw, more or less air is allowed to bypass the throttle plate. Kind of like a manually controlled aux air valve.
By adjusting the idle screw, more or less air is allowed to bypass the throttle plate. Kind of like a manually controlled aux air valve.
05-15-2006, 12:03 PM
#5
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Originally Posted by SMTCapeCod
Your second assumption is right on. It leaks air past the plate.
05-15-2006, 12:16 PM
#6
Race Car
Pushing my knowledge & recollection here....
There is a picture HERE
IIRC, the hole visible above the throttle plate has a companion, corresponding whole underneath. And the air is bypassed around the throttle plate via this channel, which is opened or restricted by the screw.....so I don't think there is any commonality with the other vacuum ports....but now I have myself all confused because such a setup wouldn't introduce air that actually bypassed the metering plate...hmmm...need a refresher...
There is a picture HERE
IIRC, the hole visible above the throttle plate has a companion, corresponding whole underneath. And the air is bypassed around the throttle plate via this channel, which is opened or restricted by the screw.....so I don't think there is any commonality with the other vacuum ports....but now I have myself all confused because such a setup wouldn't introduce air that actually bypassed the metering plate...hmmm...need a refresher...
05-15-2006, 12:27 PM
#7
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On the front of the TB there are two vacuum ports (at least on mine) one for the Vacuum limiter device (removed from my car) and the other feeds the ignition retard on the distributor.
So the two holes in the TB, one of top of the other below the throttle plate, are for these two lines. So I wonder, do these also "feed" the idle adjustment.
So the two holes in the TB, one of top of the other below the throttle plate, are for these two lines. So I wonder, do these also "feed" the idle adjustment.
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05-15-2006, 12:35 PM
#9
Race Car
Originally Posted by Hacker-Pschorr
On the front of the TB there are two vacuum ports (at least on mine) one for the Vacuum limiter device (removed from my car) and the other feeds the ignition retard on the distributor.
So the two holes in the TB, one of top of the other below the throttle plate, are for these two lines. So I wonder, do these also "feed" the idle adjustment.
So the two holes in the TB, one of top of the other below the throttle plate, are for these two lines. So I wonder, do these also "feed" the idle adjustment.
Ah, I see you're point (doh). I'll sit patiently with you and wait for some of the true experts to chime in. At least I kept your question at the top of the heap for a while!
05-15-2006, 03:20 PM
#10
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The idle speed screw is unrelated to the vacuum ports on the front of the throttle body. There is an opening in the body that connects the throat above and below the butterfly. The idle speed screw changes the cross section of that opening thereby changing the airflow and the idle speed.
05-15-2006, 04:09 PM
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05-15-2006, 05:13 PM
#12
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Originally Posted by Phil Silano
The idle speed screw is unrelated to the vacuum ports on the front of the throttle body. There is an opening in the body that connects the throat above and below the butterfly. The idle speed screw changes the cross section of that opening thereby changing the airflow and the idle speed.
