Choosing between mechanical- or vacuum-operated secondaries on a Holley four-barrel carburetor really comes down to not only understanding the engine’s performance requirements but also what the driver wants to feel through the bottom of the seat.

The underlying beauty of a four-barrel carburetor designed for street performance is that the front half, or forward two barrels, handles the brunt of the work—that is, cold-start, idle, law-abiding acceleration from stopped and routine, low-load cruising. Everyday driving on just two barrels improves drivability and fuel economy. When extra power is essential for freeway-ramp acceleration, passing or Saturday night passes at the drag strip, then the rear two barrels will open as needed to feed the extra fuel and air to the engine and increase power.

The basic operational theory is the same for mechanical and vacuum secondaries, but the execution is quite different—and that’s where understanding the engine’s demands is critical to making the right choice. The end user must consider the type of vehicle and weight, transmission choice, gearing, camshaft timing, intake manifold style and intended use.

“These are the types of questions you’ve got to ask yourself,” stresses Holley engineer Laura Shehan. “Another thing is, what do I want to feel?”

There are enthusiasts, mostly with manual transmissions or high-stall torque converters, who seek the neck-snapping rush of quick-opening secondaries and need a double-pumper carb with mechanical secondaries, while others focus more on cruising and showing. Those users will benefit more with vacuum secondaries.

Not being honest about the engine and vehicle setup along with the intended use can get a user in trouble. That’s because mechanical-secondary carbs, when not properly tuned, or used in the wrong application, can be temperamental.

“With a vacuum-secondary the universal application theory usually applies,” says Shehan. “With a mechanical, if your application doesn’t need the size or the response that carb is designed for, then tailoring accelerator pumps and tuning are more critical.”

How Do They Work?

Opening the secondaries is first dependent on the velocity of air flowing through the primaries. As the primary side opens, the air velocity flowing into the intake manifold increases with the engine rpm. This surging airflow provides a stronger and stronger vacuum signal to the diaphragm that controls the secondaries. As such, the secondaries open progressively as the engine requires more air and fuel to keep up with the throttle demand.

As the secondary throttle blades open and air starts flowing, an additional vacuum signal is generated to the diaphragm to keep the secondaries open as needed. However, the initial opening rate is controlled by a single diaphragm spring. The stiffer the spring, the slower the rate of the secondaries opening.

Holley offers a tuning kit with seven color-coded springs so that the user can adjust the secondary opening rate to suit the needs of the engine and vehicle. Black is the stiffest, followed in order by brown, plain, purple, yellow, orange and white, which is the lightest.

The plain or silver spring is most common in production carbs. Some carbs come with black and yellow springs for tuning options out of the box. Other Holley carbs, like those intended for off-roading, come with black springs to offer the user the safest starting point. There are application-specific carbs that will have a more appropriate spring for the intended use. Check the product details and installation instructions for more specific information on springs selection from the factory.

Holley offers tuning kits for the vacuum secondary system that includes springs of different weights. These range from the black spring, which is the stiffest, to the white spring, which is the lightest. Be sure to verify which spring is used in the original application for your carburetor...while the silver spring is most common, some carburetors utilize specific springs for the intended use. You can order a vacuum secondary diaphragm spring kit with p/n 20-13.

There are two types of diaphragm housings on Holley carbs. Changing springs in the traditional housing requires the removal of the choke housing as well as removing the diaphragm housing itself to access the spring. Holley does offer a quick-change conversion that doesn’t require the removal of the two housings, effectively cutting the installation time in half. On a side note: Quick Fuel carburetors use a vacuum secondary housing with an adjustment screw to control the vacuum signal pulling on the diaphragm, and therefore controls the rate of the secondaries opening.

Some long-time racers and enthusiasts have avoided the kit and tried cutting the spring already installed to reduce some of the stiffness. This is not a good idea.

“Everybody wants to apply their own black magic to a carburetor. People have their own theories about power valves, squirters, how to set the accelerator pump,” says Shehan. “The vacuum spring is another one of those things where certain people, in certain applications, are going to do what they would normally do, regardless of trying to influence them otherwise.”

In order to remove the vacuum diaphragm, you will first have to remove the choke housing from the carburetor's main body so that you can access all of the diaphragm's mounting screws. Be sure to be very careful when removing the small E-clip off of the diaphragm's linkage...it's tiny! If it does manage to escape, you can order a new one (p/n 36-1QFT).

The irony is that cutting a coil off the spring and stretching it out to make up the difference in length actually increases the effective spring rate of the spring and may not provide the desired effect.

The goal of changing springs is to eliminate any bogs, stumbles or hesitations during acceleration.

“You want to put the lightest spring that's going to suit your application,” advises Shehan. “You want your secondaries to open up, but not too soon.”

In other words, if the vehicle accelerates smoothly with no flat spots, or perhaps you’re not even feeling the secondaries open, then a lighter spring can be tested. However, if there is a bog in the 2,000 to 3,000 rpm range when the secondaries would normally open, then a heavier spring will likely solve that problem. If there’s a hesitation at the initial throttle opening or off-idle, then the accelerator pump may be to blame.

When changing springs, some users may remove the small check ball that’s located in the vacuum passage. Not all carbs are designed with a check ball; but if so equipped, the ball is there to help avoid sudden or unwanted opening of the secondaries. Not returning the ball to its seat when changing springs may invalidate any tuning improvements the user is trying to make.

Once you remove the four screws from the vacuum housing's cap, here is what you will have: the spring will fit onto the stud on the underside of the lid, while the diaphragm itself will be in the bowl. Note the complete hole in the diaphragm rubber: this is for the pressed in restriction, or in older carburetors, the check ball location.

“Also, when you're tuning your springs for your vacuum secondary, make sure that the air cleaner is installed,” notes Shehan. “If you don’t have the air cleaner on, that changes the dynamics of the pressure drops and that changes the vacuum signal.”

Don’t judge the secondary opening by revving up the engine in the garage. Vacuum secondaries open under engine load, so if they do open by winging the throttle then they will most likely bog the car when it’s accelerating under load. Also, do not try to run without a spring in the diaphragm housing. This could lead to very erratic idle performance.

One final tip: don’t toss the diaphragm and improvise some crude method of opening the secondaries manually. A four-barrel carburetor designed with manual secondaries will have a second accelerator-pump nozzle or squirter that adds additional fuel when the secondary throttle blades open. This design is often referred to as a double-pumper carb because there are accelerator-pump fuel nozzles for both the primaries and secondaries.

“You want that extra accelerator pump shot in place to overcome some of the bog or hesitation that you might have by not having enough fuel when those secondary plates open,” says Shehan.

This small cork gasket on the diaphragm housing is for the secondary venturi pick-up port. If this gasket is damaged in any way, replace it with a new gasket (p/n 8-9QFT).

Because they work by sensing engine load, vacuum-secondary carbs are very forgiving when compared to their mechanical secondary counterparts. That’s why an engine that requires a 650-cfm carburetor can operate reasonably well with a larger 750 cfm—because the engine will draw only as much air as it needs. But installing a mechanical secondary carb requires a more precise match to the engine. In other words, over- or under-carbureting will definitely impede performance.

“That’s why we offer double pumpers in 50-cfm increments,” notes Shehan.

Correctly sizing a double-pumper is very critical to optimum engine performance, especially on a full-race configuration with a radical cam that limits manifold vacuum. If the carb is oversized for the engine, then that extra shot of fuel may overwhelm the engine before airflow through the carb is controlling the main fuel feeds in a normal manner.

If you have a need to tune the vacuum system often or just want an easier way to access the spring, a quick change vacuum secondary housing cover (p/n 20-59 or 38-1000QFT) will get the job done. A plate covers the diaphragm, while two screws will provide access to the vacuum spring without removing the diaphragm itself.

Few double-pumper carburetors open the secondaries at a one-to-one or on an equal percentage rate as the primaries. Those carbs are generally reserved for all-out drag cars that run WOT throughout the run. Most double-pumper carbs are equipped with a progressive linkage that starts opening the secondaries when the primaries are about 40 percent open. There are kits that allow a quicker or later opening for fine tuning a road-race or dirt track car that is often on and off the throttle.

These tips apply to Holley four-barrel carburetors. There are other styles of carbs, such as the Street Demon, that uses an air valve or auxiliary throttle valve to control the secondaries.

A properly tuned carburetor with vacuum secondaries will provide excellent drivability and fuel economy when compared to a mechanical-secondary carb. It will also be more forgiving in terms of size selection and tuning flexibility. A mechanical-secondary carb will provide better throttle response but must be tuned and sized correctly or it can be fussy to live with on a street machine.

“The vacuum-secondary carburetor is going to take into account the engine needs, much more so than a mechanical-secondary carb,” sums up Shehan. “A vacuum carb responds to the engine’s actual needs as opposed to a simple mechanical linkage.”