[Tom Vaught]

A Bit more Carb PHYSICS today in my post.

1) Gasoline which comes out of the Idle Discharge Port (below the Throttle Plates) enters the air stream of the carb. When the fuel comes in contact with the air it is torn apart into a large number of very fine droplets and resembles a fine "Mist".

2) Ideally you would like that "Mist" to be 'Vaporized' or changed to a gaseous form.
Because the fuel and air mist are below the throttle plates the Pressure will be much LOWER vs Atmospheric Pressure under the hood.
This much lower pressure, as was said before changes the Boiling Point" of the mixture.
Some of the fuel particles are 'vaporized' as they absorb heat from the surrounding air.
Some of the fuel particles are 'vaporized' as they absorb heat from the Hot Spots in a heated intake manifold.
Some of the fuel particles just attach themselves to the colder intake manifold walls.

3) Imperfect Vaporization
a) You get Imperfect Vaporization when you have a "cold" intake manifold.
b) You get Imperfect Vaporization when you have a "cold" Ambient Temperature.
c) You get Imperfect Vaporization when the intake manifold vacuum is too low.
d) You get Imperfect Vaporization when the intake manifold design is poor or the Carburetor size is too large for the engine.

4) Why is a Larger Carb an issue?

a) With the same engine speed (RPM) the air charge velocity thru the venturis is lower,
therefore a lower "Pressure Drop" across the Discharge Nozzles (Boosters) in bigger "Chunks/ Blobs/ Globs/ or Drops" and those Larger Drops are much harder to Vaporize.

b) In the case of a 2-BBL or 4-BBL carb with Progressive Secondary Throttle Blades, the fact that you have fewer of the Throttle Blades open makes for HIGHER VELOCITY across the Throttle Blade(s) that are open.

c) Smaller Throttle Blade Diameter/ Venturi Diameter makes for Higher Velocity across the Venturi and Higher Velocity and Better Atomization of the fuel charge.

** The Pontiac Tri-Power with its small Center Carb Venturi 1-1/4" on a 64-65 Carb allows great Intake Charge Velocity across that carb and you still have the available CFM capability from the End Carbs. TV

d) Dual Quad Carbs on Open Plenum Intakes with all Throttle Blades opening at the same time will have the lowest intake charge vaporization and of course the roughest idle.

Manifold Design (as mentioned above) has a direct effect on how much of the fuel is not vaporized and therefore is attached to the Intake Plenum Walls. If you open the Throttles, the fuel on the walls comes off and then you need Multiple LARGE Accelerator Pumps to replace that fuel back on the plenum walls or you will have a hesitation after opening the Throttles quickly.

Enough PHYSICS for a while.

Tom V.