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.
Paul - 1972 Spider - (2)1991 164S's - 1983 308 - 2001 Discovery - 1997 F350
Last edited by kcabpilot; 04-02-2019 at 12:17 PM.