The photo does not load on my browser, but I think I know the part you're talking about. There should be three parts - main threaded adjuster, lock nut, and tiny o-ring. As the adjuster is moved in or out, it compresses or decompresses the o-ring, which in turn lets more or less air pass through. Make sure the o-ring is pliable, and not cracked. IMO it seems a rather primitive system, but it works. I've had to adjust mine at times.
The O-ring shown in your photos is not the one that adjusts the idle air/idle speed. It is probably still inside the plenum - where the assembly threads in. This photo shows the GTV6 idle air adjuster & O-ring.
When they get old the O-ring becomes hard and resistant to change. Replace the O-ring - I took my old one to the plumbing section of a hardware store and matched one up by eye. I don't think the exact size is that important - as long as it is soft and compressible.
Click on the link in my signature for a page of info about L-jet diagnosis.
Where would the o ring go? This is where the fitting gets threaded into the block. The black on the bottom of the bore is not an O ring.just soot. In the previous picture the inner fitting has a hole right through it. I don’t get how this would regulate air flow.
Well, there is supposed to be a fat O-ring in there. When you tighten down adjuster it squashes the O-ring making its ID smaller (and if you then loosen the adjuster it allows the O-ring to un-squish and the ID to open up). This varies the amount of air allowed to pass through and thus adjusting the idle speed.
Note that the throttle plate (aka butterfly) should be essentially 100% closed at idle so that the idle adjuster can do its job. If I find a throttle stop screw has been messed with by a DPO, I turn it so that the throttle plate is fully closed and then open it until the engine idle speed increases by 25 - 50 rpm (using the digital tach on my timing light). The prevents the throttle plate from being able to slam shut & possibly stick. Then adjust the idle speed (900 - 1000 rpm with a warm engine) using the idle speed adjuster.
One other item that can mess with the idle speed is the Auxiliary Air Valve (AAV). It is supposed to be open with a cold engine to increase the cold idle speed then close down after a few minutes as the engine warms up. But when the AAV gets old the bi-metal strip inside fails and they usually end up stuck partly open. I'm not sure if new ones are available - and used ones are a crap shoot (I've got about a 1/2 dozen failed ones and one that works). A manual AAV solves this problem - and it is self closing (you have to remember to close it yourself...).
Also the Throttle Position Switch (TPS) has an affect on the idle. I had a difficult time getting my idle speed below 1800 rpm. Finally checked the settings on the TPS and it was not engaging when the throttle came back to the stop position, meaning the ECU wasn't getting the message that the car no longer was in rev mode, and (though I don't know how this works) kept trying to keep the engine in its 'go' mode, instead of idle. Its a simple adjustment, but it seemed to fix the idle, along with adjusting the plenum adjustment that you are dealing with now. Maybe someone else has a more concise understanding of how the TPS and idle are associated, and can share that.
The TPS (as you correctly note - a switch, not a sensor like more modern cars) has three positions. Idle, wide open and between the two. When properly adjusted so that the computer knows the throttle is at idle the computer will do two things. 1) at idle it will follow its pre-programmed fuel map instead of trying to adjust the mixture. This tends to give a smoother idle. And B) on deceleration (coasting with closed throttle) the computer will cut off fuel delivery until the engine speed gets close to idle speed.
In this picture the o ring is in the wrong position,it belongs below the flat surface of the long tube ,as you tighten the large threaded nut it compresses the rubber o ring to constrict its inside diameter to limit the amount of air passing thru it.most people dont now how to adjust the idle speed on these cars .Alot of times the oring is so hard it will not collapse and you cannot adjust the speed at all. only after you replace the oring will it adjust correctly. Good Luck Paul
On my '85 Spider I replaced the O-ring with new ones I got from Alfa Parts in Berkeley, CA. I don't have the 2nd one around the Adjustment Tube. Unfortunately I couldn't find any illustrations showing these parts in the manual or the Bosch L-Jet Doc. I'd love to know if I'm missing that outer O-ring.
Adjusting the old O-ring didn't make much difference, replacing it with the new one got my idle under control and helped me isolate more vacuum leaks.
I noticed that the 164 & 168 guys have a very active thread about what they've been up to, and I think us Spider guys and gals need to get with the program- so I'm gonna start and see what happens!
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