many people have the misconception that opening the secondary throttle plates
sooner will provide increased performance
in quicker dragstrip times others think they must feel a kick when the
secondaries engage
still others believe that they should disconnect the vacuum diaphragm
and make the secondaries open mechanically let's discuss these points
in reverse order
first you should never attempt to convert a vacuum secondary carburetor to
a mechanical carburetor
there's no accelerator pump on the secondary side of the bowls or base plate
to support this change
all mechanical secondary carburetors utilize a pump shot to prevent bogging
when the secondaries are opened secondly vacuum operated secondaries will not
offer a kick that's commonly associated with the mechanical secondary carburetor
in reality you should feel a mild transition
as the secondaries begin to open and third the secondaries must not open
until the engine requires the additional air this allows torque to increase
along the peak torque curve performance is actually compromised less
by holding the secondaries is closed a little longer then
by opening them a little too soon also
if the vacuum secondaries open too fast it will result in a backfire
the air cleaner configuration and restriction
plays an important part in spring selection as well so
make sure to use your air cleaner when you're valuating your vehicle's
performance
after each change don't expect that you can wing the throttle
and see the secondaries begin to open if they do they're most likely opening
too soon secondaries should only open
when the engines under a load the opening rate of a vacuum secondary
system
is controlled by the diaphragm spring located
in the vacuum secondary diaphragm housing a lighter spring
will allow the secondary throttle plates to open more quickly
when compared to a heavier spring a spring assortment kit
Holly part number: 20-13 is available
to help you customize a secondary opening rate to your application
a very helpful upgrade part is the quick-change kit
Holley part number 20-59 it consists of a two-piece secondary diaphragm housing
cover
which after its installed can easily cut in half the time required
to change the secondary spring when making changes
do not clip a spring in an effort to make a spring lighter
so that the secondaries were open sooner strange as it may seem
clipping Springs shorter increases the spring rate
and will actually delay the secondaries opening
changing the secondary spring should never be done unless the accelerator
pump nozzle system
has been completely tuned first heavier vehicles with small engines
will require a relatively stiff spring to prevent the secondaries from opening
too soon
and causing a stumble lighter cars with larger engines will tolerate a much
lighter spring
because rpm increases more rapidly to tune the vacuum secondaries you need to
change the internal spring to a different tension rating
if you like you can click now to see details on how to switch Springs in the
standard type vacuum unit housing
models with the quick-change unit will be much easier to change Springs
there's no need to remove the entire vacuum unit during reassembly
and the diaphragm won't need to be refit to the vacuum unit when it's put
together
with the quick-change unit two screws are pulled the spring is swapped on the
cap and then the cap and screws are replaced
this change can be done very quickly with the carburetor installed in your
vehicle
when tuning the secondary spring all testing should be done
in high gear to maximize engine load if there's no stumble in the high gears
there won't be any in the lower gears you wanna go through careful trial and
error testing
over a measured distance of acceleration so you can determine which
spring will provide the optimum
opening time the best situation to use is the lightest spring possible
that will allow the secondary actuation without causing
an engine stumble a good way to verify if the secondaries are even opening
is to attach a paperclip to the vacuum secondary diaphragm rod
push it up against the vacuum diaphragm housing and take the car for a drive
if the secondaries are opening the paperclip will be pushed down away from
the housing when you check it
if not try swapping to a lighter spring