Vacuum Operated Secondary Throttles; Vacuum Operated Secondary Tuning - Holley 0-80508S Installation, Tuning, And Adjustment Manual

VACUUM OPERATED SECONDARY THROTTLES:

Many people have the misconception that opening the secondary throttles sooner will provide increased performance and quicker drag
strip 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.
Before going any further, lets discuss these points in a reverse order. First, if we could make our vacuum operated secondary
carburetors perform better by opening the secondaries mechanically, it would be to our advantage to do so since all that vacuum
actuating hardware is expensive and requires much time and money to calibrate. Mechanical secondary carburetors all utilize a
secondary pump shot to prevent bogging when the secondaries are opened. Secondly, those who "feel" a kick when the secondaries
engage are actually feeling a flat spot during initial acceleration, because the secondaries have already begun to open and have
weakened the fuel delivery signal to the primary boosters. The engine is struggling to increase speed and what they actually feel are
the secondary nozzles "crashing in" as the engine finally reaches the speed where it provides the proper fuel delivery signal to primary
and secondary venturi. Third, opening the secondaries early causes the situation described above. The secondaries must not open
until the engine requires the additional air. This allows torque to increase along the peak torque curve. Performance is compromised
less by holding the secondaries closed a little longer than by opening them a little too soon. If the opening rate of the vacuum operated
secondaries is properly calibrated there should not be a "kick", only a smooth increase in power should be felt.
In general, heavier cars require stiffer secondary diaphragm springs than light cars. Air cleaner configuration and restriction plays an
important part in spring selection also, so be sure to use your air cleaner when evaluating your vehicles performance after each
change. DO NOT expect to "wing" the throttle and see the secondaries begin to open. If they do they will probably open too soon.
Secondaries should open only when the engine is under a load. DO NOT clip or trim a spring in an effort to make a spring lighter so
that the secondaries will open sooner. Strange as it may seem, clipping springs actually increases spring rate and will delay opening.
So in order to tune the secondaries follow the steps below. A secondary spring kit is available from your local Holley® retailer (Holley®
P/N 20-13).

VACUUM OPERATED SECONDARY TUNING:

1. Remove the choke cap and choke housing to gain access to the third screw retaining the vacuum diaphragm to the main body
(Figure 14).
2. Remove three screws attaching secondary vacuum diaphragm to the main body (Figure 15).
Figure 14 Figure 15
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