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Discussion Starter #1
The accepted (and observed) wisdom is that the "squishy o-ring" idle control system on SPICA cars doesn't work very well or at all. The towers are also often cracked from over tightening the pinch bolt (mine was). This leads me to wonder about redesigning the tower to actually provide some control over the idle. Lots of people, myself included, deal with a SPICA idle that isn't where they want it, and a big difference between cold idle when the long rod / gap is sending air thru the throttle plates, and hot idle when the throttle plates are presumably closed and the tower handles idle air.

I will repeat the mantra that the "idle stop screw," mixture adjustment and advance are not the way to control idle. However the problem is that leaves effectively no way to control idle. My belief is that if there was a way to control idle a lot fewer people would monkey with the forbidden adjustments. I myself am guilty of using a bit of advance/retard fiddling.

I'm trying to think how one might redesign the tower either as a retrofit - improve on the squishy o-ring, or as a clean sheet of paper, ie a 3D print or similar.

I do understand there was a Tech Bulletin with a retrofit (anyone care to post?) but read several threads that saw little change with the retrofit. My tower is drilled to the larger size per the bulletin.

At minimum maybe people can post here about which if any O-ring setup has lead to any actual ability to control idle ?
 

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I've found that the o-ring works but it's a tune-up item just like points, cap, rotor and plugs that needs to be replaced periodically. As far as a redesign I suppose a spring loaded needle valve like every other fuel system on the planet uses would work pretty well.
 

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Discussion Starter #3
Paul is there a trade size O-ring that you've found works?
 

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Here's SB 29 from 1975. I don't think it helps very much. Even with the system opened all the way up, I don't see it helping the idle materially. YMMV. Might be possible to use a valve in the hose between the idle air tower and the airbox to modulate the idle air. I haven't tried it my self. I've just accepted the 600 rpm idle that I get.
 

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Discussion Starter #5
Thanks for posting John. Whether or not it helps its 'good' to at least get the factory's confirmation that this is in fact a problem area and not something that has developed over time from pumps/TAs wearing out etc.
 

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Discussion Starter #6
The sink washer part of the hardware store has a number of interesting options. My car never had an original ring so I don’t know which if any of these seems like a possible fit. The two on the ends I realize are a stretch the green is a number 6 AC O ring and the beveled washer on the far end would probably have to be opened up on the inside diameter some

IMG_4662.jpg IMG_4665.jpg IMG_4664.jpg IMG_4663.jpg IMG_4666.jpg
 

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This was a recommended "fix" 40 or so years ago. I did it on my '71 1750 Spider and have to look & see if I still have the valve around.

...... Might be possible to use a valve in the hose between the idle air tower and the airbox to modulate the idle air.....
 

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Hmm, my 74 Spider's idle has been going up to 1100-1200 when warm and there's no adjusting it down. What's the chance that this might resolve this problem?

Thx
 

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Discussion Starter #9
According to the test I just did: a good chance!

My PSA: try a fresh 0-ring. The "#90" o-ring 1/2" OD 1/4" ID fit nicely and offered a good linear adjustment. I have set up my car somewhat meticulously and have been chasing a good balance of idle and mixture for a while. I had the pump rebuilt by wes, set the throttle stops with an actual Alfa Romeo tool, set the pump gap hot, replaced the rod ends, adjusted the throttle cable, replaced and re-gapped the plugs, replaced the wires, used a 123 Distributor, 40kv coil and actually rebuilt the throttle bodies with new seals and bearings. It nonetheless seemed like I was getting more air into the motor at hot idle than I should be and based on tonight's fiddling I believe the little O-ring played a substantial role.

Without a doubt, it does its job of throttling the flow of air. I don't doubt it may harden and cease to do so in the near future but I think any SPICA fiddler would be remiss to write it off entirely, as I did until today.

My next McMaster-Carr order will include https://www.mcmaster.com/1173n202
 

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Hmm, my 74 Spider's idle has been going up to 1100-1200 when warm and there's no adjusting it down. What's the chance that this might resolve this problem?

Thx
With the engine at operating temperature, look at the gap between the throttle arm on the back of the injection pump and the small reference screw. It should be very small (.019") or less.

If more, then either the TA is out of adjustment, or the TA is going bad.
 

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The crushable o-ring, IMO, is a rube-goldberg setup. A much easier and more accurately adjustable method would be a small ball valve in the rubber tube between the airbox and idle air distribution tower.

You might try just disconnecting the individual cylinder tube at the air distribution tower and see if that makes any difference with zero restrictions by the air tower.
 

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Discussion Starter #12
Agree the o-ring is funky. But in the interest of changing fewest things necessary for a result im going to give it a shot. A ball valve that either threads into the tower or has barbs both sides is an idea (below for rough idea) but does make it almost too easy to play with a setting you hope is something you touch bi annually if that and will look clunky. Trying to think if there is a solution more precise than a silicone o ring that would live entirely in the tower.

IMG_4719.jpg
 

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Yes a spring loaded needle valve is what everyone else has used for idle air circuit control for as long as I've been around. It can be found on everything from a carburetor to the CIS mechanical fuel injection on my 308. The electronic injection systems usually use an electrically controlled solenoid in a bypass circuit around the throttle. The o-ring is just the most dead-nuts simple solution Alfa could come up with.

With no o-ring installed the idle air manifold "tower" as it's been called here, should pose no restriction and you should have a high wandering idle around 1200 rpm. So anything you can stick in there that will create an adjustable restriction by squashing it with the nipple will give you idle control. I once had a short length of vacuum hose in there that worked for awhile. The problem is that, unlike a needle valve, this solution won't remain stable. The o-ring is a perishable item that needs to be replaced periodically and depending on where you got it (Alfa Romeo or Ace Hardware) it probably has properties that may or may not be able to endure the thermal cycles it experiences. FWIW I'm pretty sure I have an Ace Hardware o-ring in mine and it's been working for awhile but honestly can't say for how long.

Something else I might add: We have grown accustomed to cars that you just drive for 100,000 miles and never look at. No such thing as a "tune up" anymore. The Alfa is old school back when people worked on their own cars in the driveway and you were accustomed to doing this ritual every 10 or 20 thousand miles. Pull out the distributor, clean and oil the advance weights, replace the points, condenser and cap, clean or replace the spark plugs, air and oil filter and then finally replace that o-ring and adjust the idle. It was just normal routine maintenance and enthusiast owners enjoyed doing it.
 

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Discussion Starter #15
Paul everything you say makes sense. The soft durometer silicone (hi temp, oil rated) o-ring I linked to was me looking for a slight improvement to Alfa's design.

I know and love the solenoid Idle Air Valve from Motronic cars but its been decades since I've had a CIS car. Can you post a pic / link / description of the orientation of the spring loaded needle valve that controls idle air? Pics of your 308 wouldnt hurt either :)
 

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...Can you post a pic / link / description of the orientation of the spring loaded needle valve that controls idle air? Pics of your 308 wouldnt hurt either :)
It's part of the throttle body and although the Alfa has, in effect, individual throttles it should be no problem to use a single source control as that's what they are doing anyway with the 0-ring. The needle valve is something you set and never touch again. The 308 is a lot of fun and a long-time bucket list item but I have no intention of letting my old Spica injected Spider go. I need one of each :001_cool:
 

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Discussion Starter #18
Do I have this right? Beautiful car. IMG_4795.jpg


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No the needle valve is an integral bypass in the throttle body (all internal) The hose connection I believe has to do with the Warm up Regulator that is located on the right side of the intake plenum and is another metered air bypass device that is only active on cold start and warm up.
 

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Discussion Starter #20
Spent some time tuning the GTV today, progressively leaning out the mixture. Prior to installing the O-ring I couldn't get anywhere near 9 turns in as the car would die at idle. Being able to control idle air has gotten me to 9.5 turns with an extremely nice idle. Turning the idle air tower screw absolutely modulates idle. I believe this mechanism has been maligned and should not be ignored by SPICA tuners.
 
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