How do 4 barrel carbs work

with progressive linkge? I understand 2 barrels open when you press on the pedal, the the second two open later, at full trottle? Do the second 2 barrels open by mechanical means or by vaccum! How can I test to see if the second set of barrels are opening? cb

Both kinds exist. If it's a mechanical secondary, you can just open the throttle and the secondaries typically open when the primaries are about 40 degrees open (this according to Holley - your brand may differ somewhat).

What kind (brand, model) of carburetor do you have?

I have believe it's a Holly-will check. When I move the linkage by hand, the front two battel butterflies open, but not the back. This is with the engine not running. Maybe I should have the engine runnin and see if the back two barrels butterfly opens? Not running the back butterflies do not open even with full open throttle. Link just "slips"in the semi-circular attachment to the back butterfly shaft. This made me wonder if the back butterflies were activated by vaccum rather than hard linkage.
Will check tomorrow pm. cb

Chip,

You have vacuum secondaries. With the engine
running, and the throttle opened enough, the
engine will pull enough vacuum to open the
secondary butterflies.
Throttle response is a little bit faster with
mechanical secondaries, but fuel mileage is
horrible.


Bill

On the street vaccumme secondaries work IMHO better. On the track at 8/10 they work fine as well.

Thanks everyone. Thoufgr was only running on 2 barrels and when if the seconds 2 barrels were not kicking in, would have a world of power in store to tap into! cb

And you can have the best of both worlds by employing an old trick from the drag racing wars: Find a sheet metal screw of just the right size and screw it right into that slot which you referred to and which allows the secondary linkage rod to sort of slide without opening the butterflies. Put the screw in the right position to prevent the sliding action thereby making the secondary linkage operate mechanically rather than off of the vacuum. When you're done racing and you want the secondaries to operate by the vacuum you simply remove the screw.

[ October 24, 2002: Message edited by: BlueOvalBlood ]

In response to BlueOvalBlood, that wasn't a drag racing trick. What it really turned out to be was people not knowing much about the carb on the engine. If you tuned your carb for the engine, the secondaries opened right when they were suppose to, and closed when they were suppose to. I ran into people who didn't know the secondary vacuum diaphram was not holding a vacuum, that why the secondaries wouldn't open. Then there were people who put a high lift cam in the engine, the engine wouldn't idle below 2000 rpm, because they didn't change the power valve in the carb. Get yourself some books on carbs, like the one(s) you plan on using,life will be easier.

mkast - Quote "...that wasn't a drag racing trick. What it really turned out to be was people not knowing much about the carb on the engine."
Sorry to be in disagreement with you, but my source for the post on the secondary linkage modification is "Performance Tuning For the Restorer - Fords of the 60s", a book from the Hot Rod Magazine technical Library series. On page 71 in the chapter entitled "Engine Tuneups", written by the renowned Ford engine tuner Les Ritchey, is a picture of a Holley carb similar to a 600 to which the caption reads "Four barrel Holley carburetor with modified linkage, making possible mechanical opening of secondary butterflies which are normally controlled by a vacuum diaphram." The author goes into a great deal of detailed info regarding how to set up the carburetor for drag racing, circle track, and mileage optimums. In the lead picture the caption reads "Author Les Ritchey is quite experienced at setting up Fords for drag racing. He's seen here making practice run at Pomona, Calif." I don't really think that Les Ritchey would have been considered to not know much about the carb on the engine.
Best regards, Blue

Blue,
With the different assortment of springs available, and now with an adjustable diaphram cover, I just can't believe anyone would place a screw in the lockout linkage on the holley sitting on top of the engine costing how much? Hmmm, this is a GT 40 Forum, "Nice screw in your carb, is that stainless, no it's rusted!" I noticed the screw method isn't in the printed holley material, maybe they don't know about this "racer's trick". OK, a guy gets published, that doesn't mean too much to me when it's "backyard hack" technology. I woould like to think that when people spend the time and money to produce a piece of artwork, we can go the extra inch to keep "that screw jammed in the carb" out of the artwork.
Here comes the disclaimer, I don't mean to upset anyone, I'm sorry if I did. I guess I feel the same about this as I do about left foot brakers, an I'm sure someone on the plant was published extolling the sanity of left foot braking.

mkast - Who's taking offense? I am simply passing along some trivial info from the past. You are absolutely right about jamming an old rusty screw into the carb. I wouldn't do it to my carb. Well, not my original 427 carb anyway. GT40 - Piece of artwork or old race car? Depends on how you look at it I guess. Maybe both. To me it will be just a fun car - fun to build, fun to drive. That's all I am hoping for. I'm not interested in owning a piece of artwork. Besides I couldn't afford it anyways.
Best regards, Blue

a) All car components are designed to work within specific design parmeters. Use them outside these parameters and you will need to modify them.

b) Dancing on the brakes with both feet (and that includes left foot braking) is the quickest way around the race track. No ifs! No buts!

NB. I have no opinion about who is correct about the "screw".

Don't stick your face over your carb unless you are wearing a good pair of safety glasses or a full face shield. What if your engine were to back fire back through the carb? Maybe better yet would be to use a hand mirror to look down a running carb?

All-I have a thin Holley tuning manual, which as I have discovered, is right on about what to do and what not to do to a Holley to get it to run right. They very clearly state do NOT use the screw trick. The vacuum secondary carb is designed and tuned for specific operation and transition to secondary operation based on amount of vacuum the engine is generating. Putting a screw in the works simply introduces more fuel than the particular engine needs at the point of opening, hence wasted fuel, and a poor transition from primaries only to primary and secondary operation. A mechanical secondary carb is designed for this transition. If you want the secondaries to open quicker, as an earlier post stated, experimenting with different vacuum springs would be a more effective solution. I also agree with the earlier comment-get books, read books.

Thanks-Jack

Are you sure that it’s engine vacuum opens those secondaries? I think there's a pretty good chance that you're confusing what pulls the secondaries open on those vac-sec Holley carbs.

The whole intent and purpose for the vac-sec on these carburetors is to improve driveability in street-cars and heavier vehicles. It accomplishes this by opening the secondary throttle plates in such a way that the fuel metering signal is not abruptly interrupted by the transition of going from airway velocity of two partially open throttles to four throttles quickly whacked wide open.

Here’s a scenario: You’re sitting at a stoplight in your beloved LTD station wagon with its vinyl bench seats and 4bbl 460 inch powerplant. The engine is happily loafing at 800 RPMs, and there’s about 17 inches Hg of intake depression. Suddenly, the light changes, and you drop the hammer! Depending on which 850 cfm (for sake of discussion) carburetor you happen to be sporting, a number of things are going to happen.

First off, that 17 inches of depression is going to go away REAL QUICK. A big gulp of air is going to rush through the open throttle bores into the intake runners faster than you can think. Now the intake runners are suddenly FULL, and there is no inlet depression. Now what? While all that’s happening, the carburetor is having to transition from running on the idle metering circuit to running on the main metering circuit. Despite what a lot of people will try to convince you, ‘engine vacuum’ does NOT pull fuel into the engine. The depression formed within the booster venturi as air passes through it (in most carburetors anyway, some don’t have a booster venturi) creates a pressure differential between itself and the fuel bowl. This DP causes fuel in the fuel bowl which is at a higher pressure to flow through metering orifices or a ‘jet’ to the area of lesser pressure, which is at the discharge nozzle(s) within the booster venturi. This metering signal is a function of the mass airflow through the venturi, not the depression that may or may not exist within the engine inlet.

Anyway, that big gulp of air will cause the A/F mixture to lean excessively causing the engine to misfire/stumble while the engine tries to accelerate, unless you take steps to prevent that occurrence. Carburetor makers do this by implementing an accelerator pump to provide an adequate amount of additional fuel to aid the carb/engine in transitioning between the idle metering circuit and the main metering circuit. In addition to the accelerator pump, most carbs found on automobiles will also make use of some sort of power enrichment circuit. This is typically done by using a diaphram operated orifice (power valve) or spring/piston operated metering needles to allow more fuel to flow into the main metering circuit in addition to what flows through the main metering orifice or jets. These sense the intake manifold depression and operate based on engine load (a loaded engine having higher intake pressure than an unloaded one at idle, part throttle cruise, or deceleration.) When your foot is no longer planted on the floorboard keeping the throttle plates pinned open, engine ‘vacuum’ returns, causing the power enrichment circuit to close. Thus, the main metering circuit takes over again until such time as the throttle plates close far enough to cause main metering circuit signal to decay, and creating a depression across the idle circuit nozzles and fly-holes.

Somewhere within all this mess are a couple alternative means to an end. Whereas a mechanical secondary carburetor will use accelerator pumps on both the primary and secondary sides, a ‘vacuum secondary’ carburetor feature an accelerator pump on the primary side ONLY. Why, you ask? Because it uses a spring opposed operated diaphram to open the secondary throttle plates. This diaphram begins to open the secondaries as mass airflow through the primaries allows, thus ensuring an adequate metering signal across all four main metering circuits and eliminating the lean-stumble/misfire/backfire that would otherwise require an accelerator pump circuit on the secondaries as well.

Because you don’t have another accelerator pump squirting fuel every time you blip the throttle, fuel economy tends to be a lot better in a vac-sec carb. Heavy cars with tall gears don’t allow an engine to rev quick enough for mass airflow to quickly reach a point where the main metering circuit can react to the metering signal. So, they need an extremely LONG accelerator pump stroke while the engine accelerates to the point of making a stronger metering signal. Or, you can use the above-described vacuum operator to slowly open the secondary throttles to match the ability of the engine to consume enough air to maintain a strong metering signal.

Does this make sense? I left out the air bleeds and trims to avoid unneeded confusion. Point out any confusing points, and I’ll do my best to elaborate.

Brian Kennedy

[ November 20, 2002: Message edited by: jetenginedoctor ]

Jet
Intersting. I converted my MK-IV which has dual quads to vaccumme secondaries which I found worked much better as I primarily drive on the street.