I don’t recommend any form of vacuum or air flow measurement for synchronizing. This subject could get a several page treatise, but I never seem to have the mood and time simultaneously. A summary version...
Regardless of your method or religion, a good result depends upon a good installation. Float levels empirically confirmed, throttle shafts aligned, throttle plates correctly installed, throttle shafts not bent, no vacuum leaks etc. if that’s all ok....
All you really care about is the perfect match at idle. Even with Balogna’s vaunted precision, the airflow and fuel match at idle are so small that measuring the vacuum will, at best, get you started. The final adjustment has to be done based upon engine behavior.
With the engine off.
Start by backing out the idle speed screw such that the cable or rod-driven carb is fully closed (we’ll call that the primary carb). Adjust the synch screw with your finger touching the shaft of the primary carb and your eyes closely watching until you feel it move. The point you are seeking is when the secondary carb has fully closed and any more adjustment of the synch screw will lift the primary butterflies off of closed. With care, you can achieve all four butterflies fully closed simultaneously.
Screw in enough idle screw to keep your car running. Start the engine. Go drive around awhile.
With the engine fully warm and heat-soaked...
Set your idle mixture screws for best idle. Note how many turns each has. If/when your mixture screws give best idle and are equally threaded out, you have achieved true synchronization. This is the goal.
Unless you got a perfect match out of the box, you’ll have to make a series of small adjustments. Your “control” is idle speed. You should make all adjustments at the lowest idle speed your engine will maintain. If your adjustments force you to increase idle speed, you’ve gone the wrong way. Undo whatever you did and go the other way.
If one carb’s mixture screws are more extended than the other, its butterflies are POSSIBLY more open. Tweak the synch screw to slightly close them, reset all mixture screws, and note if you’ve gotten the mixture screws closer or farther from matching. After a round of adjustments, attempt to reset the idle speed lower. If you can, you’re going in the right direction. If you can’t, either you’ve gone in the wrong direction, or you’ve achieved a match. Idle mixture screw adjustments tell you the most with butterflies maximally closed.
You’re done when all four mixture screws have the same insertion, and your engine can maintain a very low, smooth idle. Assuming typical Alfa street cams, I’m used to achieving a 400-500 RPM idle. That’s generally proof everything is synched. Then, I reset the idle to a more typical 700-800 RPM.
Yeah, I’ve heard all of the points supporting flow and vacuum synchro methods. For these to actually achieve a best result over the above method there’s about 50 variables in the engine that must be perfectly identical across all four cylinders. Perfect. Not very, very close. Perfect.
There’s too many variables to think that measuring only one will achieve a synchronization.
Carson City, NV
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird
You can snap roll an Alfa only one time...