DZ carb secondary power valve - Team Camaro Tech
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post #1 of 25 (permalink) Old Jan 6th, 03, 12:33 PM Thread Starter
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I noticed my 780cfm 4053 carb has an 8.5 secondary power valve as delivered from Holley. A 6.5 power valve is used on the primary side. If I've got 7-8 inches idle vacuum, the secondary power valve is probably open at idle, correct?

My symptoms are a rich idle even with the idle mixture screws at 3/4 turn. I checked the thottle plate/idle slot clearance and it is fine.
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post #2 of 25 (permalink) Old Jan 6th, 03, 12:51 PM
 
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I would agree......it is definatly flirting with it.....
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post #3 of 25 (permalink) Old Jan 8th, 03, 09:37 AM
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Neil:
Make sure when you buy your new power valve you get the 'High Flow' type.
The part number will have an additional "1" ahead of the power valve rating number, (e.g.: a 6.5" will be a #125-165 for a high flow v.s. a #125-65 for a standard flow).
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post #4 of 25 (permalink) Old Jan 8th, 03, 04:39 PM
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Ditto on the above post. They are also refered to as picture window power valves, the holes are large and square, the older style are just round holes. Use the right gasket for the style power valve you have. Neil also make sure you are running power valves that are 2 sizes below the idle vacum readings or you will have an overly rich idle and lots of problems with plugs fouling.

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post #5 of 25 (permalink) Old Jan 8th, 03, 07:00 PM
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I've been giving this a lot of thought. The factory config was an 8.5 sec PV and 8" of vac at idle and actually up to 1250rpm. Even though the manifold vac is present I'm not sure the sec PV will pass fuel to the main well. My Holley book says; <BLOCKQUOTE>quote:</font><HR>" When manifold vacuum drops, a spring opens a power valve in this chamber to admit extra fuel. This fuel flows through the power valve, through the power-valve channel restriction and into the main well. There it joins the fuel flow in the main metering system to enrich the mixture.<HR></BLOCKQUOTE> This wasn't in refrence to the secondary side of things. But, it leads one to believe that the 8.5 is too high for an engine pulling 8" at idle. Is there more to it? Do the sec throttle blades need to be cracked open before the main well actually provides fuel to the discharge nozzle and out the booster venturi? On the primary side the throttle blades are open .020" or so at idle and the transfer slot is partially uncovered. On a vac sec carb this isn't the case unless you have the throttle blades adjusted open to allow a large cam to idle. I guess what I don't know is if there has to be air flow through the sec main venturi into the manifold before the PV truly comes into play.

I've only solved a rich idle problem by replacing the primary PV, the sec has never made a difference when I've changed it. I hope my quest for knowledge isn't side tracking this and my questions help in resolving the problem.

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post #6 of 25 (permalink) Old Jan 9th, 03, 12:19 AM
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Dennis, as well as others, let me take a shot at explaining the Holley quote,

I read it as when vacuum has fallen, to some value less than the number stamped on the valve, it opens to allow more fuel to join other fuel provided from another circuit. This "collected" volume of fuel doesn't do anything until acted upon by atmospheric pressure when presented by the opening of the throttle plates.

Engine creates vacuum below the throttle plates. Atmospheric pressure tries to fill the void created. As air is pushed through the venturi, a high pressure area is created, thus draws fuel from the well and the fuel is atomized into the stream of air by the high pressure created at the venturi.

Therefore, I believe the secondary power valve would not have an effect on idle characteristics until the secondary plates are cracked(?) open.

I am not a master fuel technican, nor a physicist, just my two cents......

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post #7 of 25 (permalink) Old Jan 9th, 03, 01:43 AM
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First off, I am no carb expert. When I had my DZ Holley rebuilt years ago, it did not have a secondary PV, just a plug. I left it that way and had no trouble with the idle or mixture. So, I have to agree that the secondary PV does nothing for idle. Also, I called Jerry Mc. about how to set it up once I had the engine broken in. He suggested changing the front jets from 68 to 72. This helped a lot on response and acceleration.

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post #8 of 25 (permalink) Old Jan 9th, 03, 06:55 AM Thread Starter
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Good point about the secondary PV not opening until the thottle blades are cracked. I called the Holley tech line yesterday and they said, with my 7 inches of vacuum, to change both power valves to a 3.5. Their rule of thumb was anything under 9 inches of vacuum divide by two and 10 inches vacuum and above use a 6.5 PV. I asked them why the original factory carb was equipped with an 8.5 secondary PV (knowing the factory 30-30 cam gives about 8") and the tech guy said they used a higher PV to guard against a lean condition where the secondary throttle blades might open prior to the PV. Sounds like it's worth another call....
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post #9 of 25 (permalink) Old Jan 9th, 03, 09:39 AM Thread Starter
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I called Holley again. I got a different tech rep but the same answer - 3.5 power valves front and rear with 7 inches of vacuum. He said the 8.5 secondary power valve is definitely affecting the idle circuit with 7 inches of vacuum.
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post #10 of 25 (permalink) Old Jan 9th, 03, 06:01 PM
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The power valve is on the main circut, that's the fuel that comes out of the main fuel nozzles. I don't see how a power valve can affect idle.
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post #11 of 25 (permalink) Old Jan 10th, 03, 08:33 AM Thread Starter
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I'm learning as I go on this, but my thoughts are:

1) Idle vacuum of 7" causes the secondary power valve (8.5) to be open at idle.
2) The open power valve feeds the main well via the power valve channel restriction (PVCR).
3) The main well feeds the idle circuit via the idle feed restriction.
4) The idle circuit admits air/fuel through the secondary curb idle discharge even if the throttle plates are closed.
5) Since there are no idle mixture screws on the secondary side, there is no way to lean out rich idle mixture caused by the open power valve.

Does that make any sense or am I missing something? Thanks. -Neil
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post #12 of 25 (permalink) Old Jan 10th, 03, 09:08 AM
 
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NeilB I was running an 6.5 pwr valve up front and an 8.5 in the rear, just as yours, the way it came. I'm currently running 6.5's front and rear. The local speed shop mechs told me to run much lower, around 3.5. DJD and I messed around with the pwr valves and it didn't seem to make a difference with the 8.5 or 6.5 in the rear. I left the 6.5 in, because it was the last adjustment we made and didn't seem to effect the idle. Also as for the jets, through trial and error I''ve found the 72 size jets work best, as drdave69 recomended.
Take this with a grain of salt, I've had some nightmare idling problems that's just about fixed. Good luck!


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post #13 of 25 (permalink) Old Jan 10th, 03, 10:07 AM Thread Starter
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Thanks Jess. I does seem that everyone is running 72 jets in the primaries. Are you running the stock 76s in the secondaries?
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post #14 of 25 (permalink) Old Jan 10th, 03, 06:08 PM
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Take a look here: http://www.holley.com/HiOctn/TechSer...o/FMCTech.html
Click on "Idle System" then click the image of the carb cutaway and it will enlarge to full screen.

All the fuel that goes to the idle discharge port and the idle transfer slot has to pass through the idle feed restriction. The idle air corrector adds air after the restrictor, and mixes it into a "froth".
A plugged air corrector will allow pure fuel dropplets into the engine and things will get real rich.
I'm not saying this is your problem, just it sometimes happens when carbs sit and the fuel dries out in them. I see a lot of that on our farm equipment that sits all winter, or sometimes for years.

There is a tendency to blame the power valve right off the bat when often it's not the problem. Part of the trouble is they are difficult to test.

The power valve only adds fuel to the fuel that comes out of the main booster nozzles.
David

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[This message has been edited by davidpozzi (edited 01-10-2003).]

[This message has been edited by davidpozzi (edited 01-11-2003).]
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post #15 of 25 (permalink) Old Jan 11th, 03, 07:30 PM
 
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NeilB - I think I'm running 76's in the rear. I don't think I swapped those. can't quite remember. I don't have my book with me, it's in the car, which is still at the shop till the rain stop. I'll double check and let you know.

Mr.G

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