Owners manual

10
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, let’s 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 he 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.
VACUUM OPERATED SECONDARY TUNING:
The secondaries will not open by free-revving the engine. The engine needs to be under a load before they will open. If you are
still uncertain if they are opening, you can take a normal paperclip and clip it onto the secondary diaphragm rod and slide it up
against the bottom of the secondary diaphragm housing. Go out and drive the vehicle, making sure to get into the secondaries.
When you return you will be able to look at the position of the paperclip on the rod. If it is lower on the rod, you can tell the
secondaries opened and how far they opened. This is useful in determining if you need a heavier or lighter secondary spring.
1. Remove the air cleaner assembly.
2. Remove the 2 Phillips screws from the black vacuum diaphragm spring cap (Figure 13).
3. Lift off the cap and spring. Remove the spring from the cap. Replace with a spring from kit P/N 20-13.
4. The lighter springs will allow the secondaries to open sooner (for light cars) and increase performance.
5. The heavier springs will force the secondaries to open later (for heavier cars) and increase fuel economy.
Figure 11 Figure 12
6. The silver spring that comes in the Street HP Carburetor kit has been calibrated to give the best performance and fuel
economy in most applications.
7. Once you have selected a spring, attach it to the cap (Figure 11) by placing the small end of the spring over the center post
on the cap. Check the o-ring seal and place the cap and spring back into the vacuum diaphragm housing.