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.

I would agree......it is definatly flirting with it.....

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).

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.

Dave

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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...Dennis
"The '69 (http://chevelles.com/showroom/ww.jpg), the '96 (http://www.camaroslimited.com/graphics/memcars/96ss.jpg) our local club (http://camaroslimited.com/)"
and the "daily driver" (http://chevelles.com/showroom/DjD/dsc00016.jpg)

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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Everett "OBJECTS IN THE MIRROR APPEAR QUICKLY UPON RAPID DECELERATION"

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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Dave
'69 Z11 Pace Car, 350/4 speed, manual top, console w/gauges, tilt rosewood wheel, AM radio

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....

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.

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.
Did you call on New Years day???
David

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Check my web page for First Gen Camaro suspension info:
David's Motorsports page (http://ourworld.compuserve.com/homepages/David_Pozzi/)
First Gen Suspension Page (http://ourworld.compuserve.com/homepages/David_Pozzi/first_gen_suspension.htm)
67 RS 327 original owner. 69 Camaro Vintage Racer, 65 Lola T-70 Chev SB Can-Am Vintage Racer

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

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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Jess - Camaros Limited Nor-Cal
69 Z/28 Lemans Blue

Thanks Jess. I does seem that everyone is running 72 jets in the primaries. Are you running the stock 76s in the secondaries?

Take a look here: http://www.holley.com/HiOctn/TechServ/TechInfo/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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Check my web page for First Gen Camaro suspension info:
David's Motorsports page (http://ourworld.compuserve.com/homepages/David_Pozzi/)
First Gen Suspension Page (http://ourworld.compuserve.com/homepages/David_Pozzi/first_gen_suspension.htm)
67 RS 327 original owner. 69 Camaro Vintage Racer, 65 Lola T-70 Chev SB Can-Am Vintage Racer

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

[This message has been edited by davidpozzi (edited 01-11-2003).]

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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Jess - Camaros Limited Nor-Cal
69 Z/28 Lemans Blue

I am currently rebuilding my DZ Carb (at the kitchen table!). I am sure that this carb was rebuilt before because it did not have Holley power valves. I don't think it has the original jets either. However, the jet sizes are 72 on the primary and 76 on the secondary. The power valves were 6.5 and 8.5. I also have low vacuum (8) and my mixture screws reach the smoothest idle at around 3/4 turn. The kit that I used was a Holley 37-1539. The power valve stamps are barely legible on some of them. Therefore, I wanted to "weigh-in" here because I'm in the same boat as NeilB. I don't know which power valves to install so if someone can give the definitive answer for basic driving conditions, I'll take it. Also, does anyone know the valve sizes that Holley includes in this kit (37-1539) because 4 of the 10 valves they include are not legible and maybe I could determine what they are by the process of elimination if I knew what it was supposed to include. If not, I'll call Holley tomorrow.

Thanks,
Eric.

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69 Camaro Z28/RS, Hugger Orange w/ White Stripes, Chambered Exhaust, Endura Bumper, Protect-o-plate, Original Window Sticker

I went to a 3.5 power valve on the both the primary and secondary. I also went back to the stock 68/76 jetting. With this setup, I picked up a little bit of idle vacuum (went from 7" to 7.5") and I now get best idle with the mixture screws at 1/2 turn. The car idles great at 950 rpm but it still smells pretty rich. I didn't get a chance to test drive because of the weather.

Strangely, the stock front power valve had "55" stamped on it, which leads me to believe it had a 5.5 power valve. The secondary power valve was the correct 8.5. The carb has never been apart before. Those factory metering block gaskets sure are fun!

Eric,

I had trouble reading the stamps on the power valves too. The stamps on mine (in the carb and the new ones I bought) were located on the large face of the power valve. They are small and very faint. For example, my 3.5 power valve had a 3 and a 5 stamped on it as well as a letter (such a 'S', or 'K'). These numbers/letters are stamped around the circumference of the face about 120 degrees apart from one another.

Neil,
I think I'm going to try the same setup that you did using 3.5 valves for primary and secondary. It sounds like we have similar conditions so I was wondering what you have the timing set at. Also, does your car try to keep running after you turn the key off?
Thanks,
Eric.

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69 Camaro Z28/RS, Hugger Orange w/ White Stripes, Chambered Exhaust, Endura Bumper, Protect-o-plate, Original Window Sticker

19 degrees initial and 36 degrees fully advanced when checked with a swoopy Snap On light. My Craftsman dial-back light shows 20 initial and 40 total. My motor does not run-on or diesel. Once I get the carb close, my tuning effort will include different timing settings. The domed pistons need a fair bit of advance.

Neil,
I put in the 3.5 power valves on the primary and secondary. We'll see how she runs.
Thanks,
Eric

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69 Camaro Z28/RS, Hugger Orange w/ White Stripes, Chambered Exhaust, Endura Bumper, Protect-o-plate, Original Window Sticker

I took a short drive today. With the only changes from factory being the 3.5 power valves, I picked up a fairly large off-idle stumble when I let out the clutch and begin to load the motor. This caused terrible driveability because of the need to rev the motor more when engaging the clutch. I rechecked floats, idle mixture, and accelerator pump arm and they were spot on. I went to the #2 position on the accelerator pump cam, and this helped the off-idle stumble but didn't cure it. Do I cure this stumble by going back to a higher power valve or by working on the accelerator pump? Thanks for the continuing help.

I think you need the higher numerical power valve. That is what it is for after all.

-Mark.

Neil,
I cannot say for sure if your problem is due to the change in power valves. However, as you know, I was following this string and also put 3.5's on both sides of the carb. Here is what happened to me in the hopes that it can help.

After putting on 3.5's on both the primary and secondary metering blocks. Prior to re-installing the carb, I adjusted the accelerator pump to allow a .015 feeler gauge to be squeezed between the spring adjustment "bar" and the arm that pivots against the plastic cam on the throttle linkage. I made this adjustment while holding the throttle in the wide open position. Next, I installed the carb on the motor and adjusted the floats until gas was just coming through the site holes. Finally, I test drove the car and experienced a major lag in throttle response when I gave the car gas. Keep in mind that I duplicated your timing setting of 19-20 initial. After thinking about what the problem could be, I decided to check the primary discharge nozzle to see if it was giving the carb that initial jolt of gas from the accelerator pump so that the carb wasn't dry during the initial punch in the pedal. Sure enough, the nozzle was barely spitting any gas. I then pulled the carb again and removed the nozzle, and blew out the openings to make sure they were clear and re-installed the carb. Re-tested everything and the nozzle was shooting two clean lines of gas into the carb. Throttle response was flawless and the car now runs better than ever. I hope this info might help you find the problem.
Good luck,
Eric.

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69 Camaro Z28/RS, Hugger Orange w/ White Stripes, Chambered Exhaust, Endura Bumper, Protect-o-plate, Original Window Sticker

[This message has been edited by EricL (edited 01-17-2003).]

Eric,

Glad to hear you've got yours running good. My accelerator pump arm was correctly clearanced and the pump discharge nozzle was functioning properly. I figured it was worth changing the discharge nozzles before stepping back up on the power valves. I ended up changing the stock .025 nozzle to a .031 and this made a significant improvement. I've still got a slight off-idle lean surge, but the bigger nozzle completely cured the bog I had under hard acceleration. It's really nice to have that cured. I'm going to go to a 70 main jet on the primary side to see if that cures the remainng lean surge. It may take a 72.

-Neil