When they are opened too late (the spring is too heavy), you might not run into any drivability issues, but performance will be hampered, and the engine will feel a little sluggish. If the secondaries open too early (the spring is too light), you will experience a hesitation, because there will not be an adequate vacuum signal to pull enough fuel into the venturi especially since a vacuum secondary carburetor has no secondary accelerator pump to cover this transition. The rate at which the vacuum secondaries open can be manipulated by changing the springs in the secondary diaphragm. When the spring’s resistance pressure is overcome by engine vacuum, the diaphragm is able to pull the secondary throttle blades open. When a predetermined amount of vacuum is sensed, the vacuum diaphragm pushes against a spring. This is because the secondaries open in relation to the vacuum signal developed by the engine. A vacuum secondary carburetor tends to be more forgiving than a double pumper, since they do operate by sensing engine load. That being said, it is possible to use a properly tuned vacuum secondary carburetor on a street driven car, and make it perform equally well.Ī vacuum-secondary carburetor is well suited for use on mid to heavyweight street-driven cars that are backed by an automatic transmission. They are able to react quicker than a traditional vacuum-secondary carburetor. It is common practice to use mechanical-secondary carburetors on racecars along with many high performance applications. The linkage controlling the secondary throttle plates is designed to begin opening the secondaries once the primary throttle plates have opened roughly 40 degrees. The rate at which the secondaries of a mechanical-secondary carburetor (double pumper) are actuated is predetermined by the configuration of the secondary linkage (some secondary linkage systems are easily adjustable, but that’s another story). When it comes to the secondary system of a Holley four-barrel carburetor, the opening of the secondaries can be done by either a vacuum signal, or by way of mechanical operation. Depending on the carburetor, a four-barrel’s secondary plates will open at different times, and can be actuated by different means. These engines typically have four throttle blades (four barrel). Some engines can make do with two small blades (two barrel), but on a performance engine, a lot more air and fuel needs to be fed into the engine. Regulating the airflow into the carburetor are throttle blades that are connected to the throttle pedal. The way it works is by drawing fuel and air into the engine by way of a vacuum that is created while the engine is running. The carburetor is a wondrous but sometimes-mysterious device mounted on top of an engine that makes it run.
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