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Old 10-11-2012, 10:59 AM
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Carb question about secondary barrel control

Is any advantage to using a secondary that is controlled by a vacuum source vs. air valve?

As I understand it an air valve is a "flap" that is held shut by a spring and when engine vacuum is sufficient to overcome the spring, the air valve will begin to open allowing air to move into the secondaries. More vacuum, more open, more air.
Examples: Q-jet, Edlebrock Performer 1406.

A vacuum controlled carb will use a ported manifold vacuum source to actuate the secondaries. It will have a spring controlled rubber diaphragm.
Example: Holley 4160

It seems to me that the the vacuum pod will be more precise, but more prone to wearing out and failure. Thoughts?

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Old 10-11-2012, 11:12 AM
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From a price perspective, the Carter AVS is more expensive than the Carter AFB (of which the Edelbrock Performer 1400 series is a copy of), and the AVS is the air-valve-secondary model. I'd expect they're priced that way because the AVS is of a superior design to the AFB, but honestly I've never worked with an air-valve-secondary carb. Edelbrock sells the AVS as the Thunder series.
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Old 10-11-2012, 03:03 PM
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FYI: The manifold vacuum has nothing to do with the opening of the secondary air valve or vacuum secondary in a Holley carbs. What causes the secondary to open is the air traveling threw the venturi in a carb. Manifold vacuum drops down to almost zero when the throttle is at WOT. Secondary open on demand when the motor needs it.

Last edited by lg1969; 10-11-2012 at 03:08 PM.
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Old 10-11-2012, 10:24 PM
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In other words "venturi vacuum controls the sec on a Holley carb
and secondary venturi velocity (air flow speed) opens the secondary "velocity valve" ,,,"Air door" (Qjet) .
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Old 10-11-2012, 10:34 PM
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OK good point about the venturi effect, thank you. But back to the original question, does one method have pro's or con's?
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Old 10-11-2012, 10:36 PM
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Actually I guess the air valve method is easier and quicker to adjust when the secondaries open.
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Old 10-12-2012, 08:15 AM
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Edlebrock (Carter type) carbs are more sensitive comparing to Rochester or Holley. They have a dampening effect when it comes to secondary opening.
If it open quick it gulp too much air and cause it to bog. Edlebrock Carbs are known for that.
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Old 10-12-2012, 02:57 PM
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A good friend I know raced his 4 spd 350 73 Camaro with 410 gears down the 1/4 race track using 600CFM AFB type Edelbrock. First he bogged off the line when he was using slicks. He would have to bring the RPM at a high level so he could leave the line without bogging. He replaced the carb with a Holley 650DP and he dropped .2 sec on the 1/4 and he came off the line really hard with no bogging and with less RPM.
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Old 10-12-2012, 04:24 PM
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Quote:
Originally Posted by lg1969 View Post
A good friend I know raced his 4 spd 350 73 Camaro with 410 gears down the 1/4 race track using 600CFM AFB type Edelbrock. First he bogged off the line when he was using slicks. He would have to bring the RPM at a high level so he could leave the line without bogging. He replaced the carb with a Holley 650DP and he dropped .2 sec on the 1/4 and he came off the line really hard with no bogging and with less RPM.
Would have been much cheaper to just tune the Edelbrock rather than replacing the carb.
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Old 10-13-2012, 02:35 AM
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Quote:
Originally Posted by Silver Surfer View Post
OK good point about the venturi effect, thank you. But back to the original question, does one method have pro's or con's?
The Holley vacuum diaphragm set up is tunable. It has different springs as well as the check valve system can be modified by using a lighter/smaller check ball or a check ball seat w/more or less air bleed bypass.

Downside to the Holley system is the diaphragm is hard to replace and costs quite a bit. The diaphragm can change over time by becoming stiffer w/age and this can affect the secondary operation. Also changing the spring can be a pain unless a quick change cover is bought or made. And modifying the check valve system is a trial and retrial deal that can be time consuming.

The Q-jet is also very tunable but it's a lot less user friendly, at least until one is familiar w/the Q-jet. The air valve spring is the first thing that most guys will work with to get the secondaries dialed in. But there are other things just as important if not more so.

The secondary air valve opening is damped by a vacuum chamber. This sometimes does double duty as a choke pulloff. The vacuum reaching the diaphragm is restricted by a normally nonadjustable orifice that can be drilled out to give a quicker response. Why no one has marketed an adjustable vacuum chamber for them is beyond me.

The amount of fuel entering the secondaries of a Q-jet through the secondary pull over tubes and orifices (acts as a passive accelerator pump) before the air valve is open far enough to draw fuel up from the float bowl through the secondary discharge tubes can be altered in several ways. This can have an effect on the opening rate as well.

As for the Edelbrock Performer set up (like the 1406), the air valve opens when the amount of airflow through the secondary is greater than the closing force of the counterweighted valve. The main problem w/this is the non adjustability. That's the main reason I'd use a Thunder series carb w/adjustable secondaries.

The upside is this set up has the least moving parts.
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Old 10-13-2012, 08:53 AM
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educated foot,

My older brother had a 40 ford coupe with a merc flathead with 3-2's, and in the old days you had to know by feel and sound when and how much foot to use to open the other carbs, I modified the linkage on a few 4 barrels to be all mechanical.
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Old 10-13-2012, 11:23 AM
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On multiple carb set ups, most often a progressive linkage will work better on the street than linkage set up to open everything at the same time. If the linkage isn't progressive, the driver needs to have a good feel for what the engine needs- much like using a double pumper on the street. There's nothing wrong in particular to that- but there is a learning curve involved; not a plug-and-play deal like a vacuum secondary or progressive linkage set up.

Pontiac (and possibly others) went so far as to hook the end carbs of some of their Tri Power induction systems to a vacuum motor that acted to bring the end carbs in like the secondaries of a vacuum secondary carb. Chevy used vacuum operated Holley 2-barrel end carbs on the BBC 'Vette tripower. In both cases this was to make the system more user friendly, requiring less thought on the part of the driver.

If I had a nickle for every Holley vacuum secondary carb I've seen w/a screw in the secondary throttle arm to make it into a "mechanical secondary" carb, I'd have a at least a couple bucks.

The problem w/this is there's no active accelerator pump for the added airflow, so what results is a huge bog when the secondaries are made to open before they would normally. The primary accelerator pump can be made to cover some/most of the hole in the air/fuel ratio that results when the secondaries are opened prematurely- but then the carb won't work worth a damn on the street because there will be WAY too much pump shot under anything but a WFO throttle. Ordinary around town driving will foul plugs, the off idle response will be the pits and economy will be out to lunch.

There are mods that can be made on the Q-jet to get the secondary throttle blades to open w/less primary throttle blade angle, but this will often bring in the secondaries much more often and sooner than needed on the street and drivability and economy will suffer as a result.

Holley has 1:1 primary:secondary throttle linkage that will open both the primary and secondary at the same time, but this is used on double pumper carbs. Again, not for the street.
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Old 10-15-2012, 08:00 AM
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Quote:
Originally Posted by Greg T View Post
Would have been much cheaper to just tune the Edelbrock rather than replacing the carb.
Not when he is using a 1406 AFB style carbs. Like cobalt327 wrote, The secondary are not adjustable. It would open too quick when he floored the pedal and the result was a lean bog. With the Holley DP that problem went away.

Last edited by lg1969; 10-15-2012 at 08:10 AM.
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Old 10-15-2012, 05:02 PM
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There are multiple problems that can cause an Edelbrock 1406 to bog. Often it can be fixed by simply changing the step-up springs so that it transitions to the power mode more quickly as the vacuum drops.

The stock Edelbrock step-up spring on the 1406 will transition from cruise to power at 5" of vacuum. This may create a bog because it goes too lean when the throttle opens and the rods pop up too late. Simply switching to 6", 7" or 8" springs will make it transition to the power step on the rod sooner, which often cures the initial bog. That transition bog usually cannot be resolved by adjusting the accelerator pump, which is the first change many folks try.

I added a wideband AFR meter to my truck and it was very easy to see when the Edelbrock went lean during acceleration, and also very easy to see how it improved by changing the step-up springs. In my case a switch to 6" springs resolved the problem

I also experimented with changing the jets and rods, but this was a much more gradual change in AFR. You could see the change on the AFR meter, but it was tough to feel by seat-of-the-pants.

Bruce
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