... and more importantly than all of the below, you have to have the
right primary venturi size. Many folks think bigger is better.
While that will give a higher top end, it doesn't allow the high
airspeed through the venturi which is needed to get the fuel flowing
correctly.
Lester
On Jun 18, 2006, at 2:58 AM, Robert E. Shlafer wrote:
> Steve...
>
> The Weber is a "fixed venturi" carb
> ("open" choke) while the SU is of the "variable" choke type. "Choke" =
> "venturi".
>
> Which is why the Weber gives superior INITIAL response assuming two
> things...
>
> 1. Reasonably good engine "health"
> 2. Correct carb. calibration
>
> As Frank says, when you "stomp" on it, the
> Weber butterfly opens fully accompanied by
> a "shot" of fuel from the carb's accelerator
> pump circuit.
>
> If the engine is down on compression OR
> the accelerator pump "shot" is incorrect,
> (too little or too short a stroke or a combo
> of both) the motor will "bog" (flat spot)
> between the "idle" and "main" circuit.
>
> This is especially so with unheated intake
> manifolds....less heat = less atomization =
> requirement for more fuel per lb. of air for those milliseconds
> before the main system comes in after you put your foot in it.
>
> Sometimes going slightly richer on the idle
> jet (or slightly smaller on the existing idle jet's "air" orifice)
> will be helpful in this respect as in enriching "progression" (off-
> idle) before the main system "comes in".
>
> Or going up one size on the acc'l. pump jet.
>
> But check your compression/vacuum first and be sure the motor is up
> to snuff. For if
> not, carb "calibration" (or lack of intake
> manifold heat) isn't the problem....engine
> mechanical condition is.
>
> The SU is more user friendly with engines down on compression for
> while the butterflies fully open when you put your foot down, the
> "air valve" will
> only open to the extent of engine "demand"
> (vacuum) and no further ("variable" choke).
>
> IOW, the SU will only pass the amount of air the engine can "take"
> and no more. The rate/amount of air valve travel in these
> milliseconds draws the "correct" amount of fuel for acceleration as
> the needle rises in the jet to "pass" fuel to the incoming air
> determined by the position of the needle in the jet as the air
> valve rises (in "rate" and
> "travel") in accordance with the engine's vacuum "signal" (engine
> "health").
>
> The foregoing "mixture strength" (for
> acceleration) is determined by the viscosity
> of oil in the damper reservoir ("dashpot")
> and to a far lesser degree by the compressibility of the air
> valve's "spring",
> the latter actually controlling air "travel"
> with respect to max rpm (should be fully
> open at "redline" if spring tension is correct
> for state of tune).
>
> Cap'n Bob
> Basic Frog
>
> Cap'n. Bob '60 :{)
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