When do vacuum secondaries work

Start the engine and allow it to warm up to operating temperature. Adjust the fuel level by setting the float first. Adjust the accelerator pumps. Start the vehicle to set the mixture. Adjust the idle with the idle screw located on the throttle linkage.

How do you adjust a Holley 2 barrel carburetor? How to Adjust a Two-Barrel Holley Carburetor Locate the curb idle speed screw and the idle mixture screw; both can be found at the base of the two-barrel Holley carburetor.

Remove the rubber vacuum hose from the vacuum port on the side of the carburetor. Will drinking salt water make you crazy? What is internal and external criticism of historical sources? Note the complete hole in the diaphragm rubber: this is for the pressed in restriction, or in older carburetors, the check ball location. Also, do not try to run without a spring in the diaphragm housing. This could lead to very erratic idle performance.

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. This small cork gasket on the diaphragm housing is for the secondary venturi pick-up port.

Because they work by sensing engine load, vacuum-secondary carbs are very forgiving when compared to their mechanical secondary counterparts. 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. 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.

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. So you've acquired a project, open the hood and find there's no carburetor or throttle body bolted atop your new engine.

The secondaries should be the last thing you tune. Floats, then Idle, then primary jets, then secondaries. As the previous system effects the next system when it engages. Did the paper test and the paper is still sitting there. So the secondaries are not opening under load.

Is there anything else I can eliminate? Like i said the carb is running great. Idle seems to be dead on. Vacuum is at 14psi and at 5k alltitude I'm told that's great. I'm going to try calling Holley and see if i can get a hold of someone there.

I've never seen a v8 that couldn't open any of the lighter springs. You say you hear the hiss when you manually open the secondaries, but where is the air escaping from?

Likely that is the source of the problem. Also it would be a vacuum leak at wot so that would lean it out as well. It is either the vacuum cover over the diaphram or the seal between the whole diaphram canister and body of the carb.

If it's not from the port in the main body not sure where that should be but I thought it was a venturi then the sound of air outside the main body would be trouble. I did have a Holley cfm 3 barrel on my that never opened the secondary gee, I wonder why! I called Holley tech support yesterday and they said the only thing it could be is a clogged vacuum port.

I'm going to pull the carb and try to clear all the passages. There is a test where you force air through a line in the primary section of the carb not quite sure what he was talking about, but i'll figure it out and it should open the secondaries. If it works then that part of the carb is fine and the only thing it could be is a clogged line. I just find it wierd that for as clean as this carb is something like this could be happening. I should know later today what's going on.

After be frustrated these past few weeks not being able to figure out why a perfectly good carb wasn't working right, I decided to take the carb apart agian for the XXXXX time. I found out that the secondary vacuum port I think that's what it's called was not clogged. When I put everything back together and I tried to see if air was being forced out by the diaphram, I realized that the only sound I was hearing was the suction of the diaphram.

At this point I was about to throw it across the room. Something in my head told me to turn the carb upside down and test it again. Low and behold it worked. So I thought to myself what is the only free moving part in this carb. The check ball. Since nothing to this point had worked I decided to use a smaller check ball two came in the rebuild kit-one the same size as what was in my carb and the second one a little smaller then the one originally in my carb.

The carb works fine now. I'm not sure that this was the way to fix this problem, but it works so I won't complain for now. I still have a lot of tuning to do and will probably be back soon asking more questions.

You can post now and register later. If you have an account, sign in now to post with your account. Paste as plain text instead. The barrels are the openings in the main body of any carburetor. These are more than a simple opening for air to flow through, though. A venturi is a narrow passage that works to increase air speed. As that air speeds up, pressure drops and creates a vacuum that works to draw fuel into the air stream.

The size and count of these barrels correlate with how much air and fuel are supplied to the engine. Each barrel on the carburetor begins to supply air and fuel as the throttle blades underneath start to open. Not all four barrels open at the same time on most configurations.

In most cases, you have two blades open first. After so long, the second set will open. As you can guess, the first sets to open are your primaries, and those that open later are your secondaries. Out of the gate, we need to mention that comparing carburetors with mechanical and vacuum secondaries tailors the discussion to Holley carburetors or those based on similar designs.

Mechanical secondaries are the easiest to understand since the operation mirrors that of the primaries. As we said, each of the barrels on your carburetor supplies air and fuel when the throttle blades are open. The operation of the primaries on most carburetors is achieved through a mechanical linkage attached to the throttle cable. As you depress the accelerator pedal, that cable pulls the blades open.

With a mechanical secondary carburetor, the linkage on your primaries is attached to a mechanical connection on the secondaries. The link is reliant on a connecting rod that moves through a slot on the secondary linkage.

As the primaries open, that rod will slide through the slot until it is stopped and then begins to pull the mechanism, opening the secondaries. Remember when we talked about how the air in the venturi builds vacuum? It simply allows them to open.