This kit includes a selection of secondary diaphragm springs to tailor the secondary opening characteristics to a particular engine
and vehicle configuration.
Many people are of the opinion that opening the secondary throttles sooner will provide increased performance and quicker
dragstrip times. This is true, as long as there is sufficient airflow when the secondaries open to pull fuel from the discharge
nozzles. Those who feel a “kick in the pants” when the secondaries kick in are actually feeling a flat spot during initial
acceleration. This happens because the secondaries have already begun to open and have weakened the fuel delivery signal
to the primary boosters. The engine struggles to increase speed and what they actually feel are the secondary nozzles
“crashing in” as the engine finally reaches the speed where it pulls enough airflow to provide the proper fuel delivery signal to
the primary and secondary venturii. Chances are that they will find the car faster from the start to the “kick in” point, if they
disconnect the secondaries.
Before starting modifications, it is a very good idea to obtain a stopwatch. Small gains in acceleration cannot be felt by the seat
of the pants and the roar of the engine can deceive even an experienced tuner.
TUNING:
First, make some notes about carburetor performance as it
is in stock condition. Does the engine
falter or stumble
at wide-open throttle,
anywhere in the rpm range? Can the
secondary opening point be distinctly felt?
If there
are no flat spots or stumbles,
a lighter secondary
spring should be installed. Try the
accelerations again and if there
are still no flat spots, try a lighter spring yet. When a flat spot is felt, go back up to the next stiffer spring. If the carburetor bogs, out of the box, note when it bogs. If it is an
instantaneous off idle bog, the accelerator pump may be the real problem. A secondary
bog will usually occur in the 2000 to 3000 rpm range. If this is the case, install a heavier diaphragm spring.
The first
thing many people do when
they first disassemble the diaphragm
assembly is throw out the small metal ball that is contained in the vacuum passage
in the lower housing, without really knowing
why it is there. At first
glance, it looks as if it would
block the flow of air, but a close
inspection will reveal
a small groove
in the seat under the ball. This groove acts as a restriction, so that the
secondaries open in a slow controlled manner.
Removal of the ball will allow the secondaries to more or less flop open and may cause a bog, so unless your driving is
restricted to the track, leave the ball in place.
Some of the newer carburetors replace
the ball and seat with a pressed-in brass restriction. Drilling
out this restriction will have the same
effect as removing the ball.
In general, heavy cars require stiffer
secondary diaphragm springs
than light cars. Air cleaner
configuration and restriction plays an important part in spring selection, so be sure to use
your air cleaner when evaluating your vehicle’s performance after each change. An installation with an open element air
cleaner will require a weaker spring
than one with a restrictive snorkel-type air cleaner.
Vacuum secondaries are designed to open when the engine
is under load. “Winging”
the throttle with the transmission in neutral should
result in no secondary movement. If
they do open in neutral, the engine will almost surely bog under load. Do not clip a spring in an effort to make
a spring weaker, so the secondaries will open sooner. Strange as it seems, clipping springs actually increases spring
rate. While the spring load as installed will be lower, the spring’s resistance to change
in height will be greater,
which means that the secondaries will start to open sooner,
but will reach
wide-open position later.
Regardless of what you hear do NOT leave the diaphragm spring
completely out. Although
the link on the left side of the carburetor will force the secondaries
to close, the diaphragm spring ensures complete return to idle giving the
engine a consistent idle speed.
If the lightest spring is installed and the secondaries still do not open when all parts are functioning properly, the carburetor is most likely too big for the engine and forcing the
secondaries open by mechanical means will probably slow the car down.