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 :{)
|