Push hard and live
Join Date: Apr 2007
Location: Carson City, NV
The FBV does flow air, not just compensate for pressure. The upper end of the emulsion tube carries an "air corrector" jet that accepts the air that is used to emulsify the fuel. Not a lot of air compared to the main flow into the engine, but some.
Keith Franck and a few others are spending a lot of dyno time, and using AFR meters while driving around to perfect their concepts. As I understand them (and I may not) there are two main goals: remove all dips and peaks in the mixture as the RPM/throttle changes trigger transitions from one circuit to the next, AND to eliminate the need for a small primary Venturi to amplify the signal to the secondary Venturi.
It might jar everyone loose from "that's the way Alfa always did it" to tell you that Keith has abandoned the concept of emulsifying the fuel. This is a fundamental change from Weber's core concept.
In spite of it being such a major change, installing his VF tube using my existing main and air correction jets improved the mid range immediately, with no other change to the jetting. Keith has told me he sent his VF5 tube, which he was sure would be too rich. From here, I'll stick an Innovate AFR sensor up the exhaust and start swapping VF tubes until my mid range mixture flattens out. Once I've gotten the driving AFR looking good I'll go back to the dyno.
Keith's extensive sampling of Weber equipped cars is that the transition from the idle circuit to main goes tragically rich, causing the common big slump in that range. Note that he has quite sophisticated and adjustable idle jets to participate in seeking perfection.
A common cause of retrofitted Weber applications falling over is due to using large venturis. This is apparently caused by creating an insufficient boost in the signal to the secondary Venturi. I'm trying to imagine what my 2300 is going to produce if I can switch from my 35mm venturis to 40s with no loss of low RPM torque.
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...