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I am by no means an expert on carburetors. Recently I have taken on an engine build of my lifetime. As this build progressed, I have learned about things I have never even heard of. One of these things is "referencing the power valve." The power valve in your carb operates on vacuum, or should I say a lack of vacuum. Blower carbs are available and already have the power valve referenced. I had already purchased mine before knowing anything about this so I tackled this as part of my learning curve. This project was a crash course for me. All the info in this article has been gathered on various web sites and tech boards. To the best of my knowledge it is correct. If anyone can add or correct me please feel free.
A referenced power valve carburetor is intended for use with superchargers, and have the capability that allows their power valves to reference the intake manifold vacuum below the supercharger. This will allow the power valve to operate as it is designed, based off intake manifold vacuum. A power valve provides further enrichment to the main metering system under load (low vacuum) conditions. Without this external referencing, the power valve would be "reading" the Supercharger boost pressure, which has no bearing on the engine load.
The following was taken from Holley's tech site:
Nothing will kill a blower or Nitrous engine quicker that a lean condition. You want plenty of fuel available for the engine to use .There is a thing you need to know about the power valves on a roots style blower engine. The power valve is installed to keep the engine from loading up and running rich at an idle. On a normally aspirated engine the engine vacuum at idle will hold the power valve closed. When you step on the gas the throttle plates open and the engine vacuum drops as you accelerate. When the vacuum drops below the rating of the power valve , it snaps open and richens up the main system. On a blower with the carb mounted above the rotors there is constant vacuum all the time even under wide open throttle. The power valve will never open and you will have a lean condition. To remedy this there is a modification you can have done that is called manifold referencing the power valve. You plug the vacuum feed hole in the baseplate for the power valve. Then you drill a hole in the side of the main body into the hollowed out vacuum chamber for the power valve. You then insert a vacuum nipple in this hole. You will run a vacuum line to the lower intake manifold from the new vacuum nipple. Now you will have vacuum on the power valve at an idle, and when you hit the gas as the boost builds, it will force the power valve to open and richen up the main system. This can be done by most carb modifiers or even yourself.
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Step 1.Find the power valve port. This base plate is a ProForm. This is a Holley design and should be very similar if not exact depending on the model. Overall, the end result will be the same.

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Step 2. Drill out the brass insert and remove the check ball ( power valve protect models)

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Step 3. I used a wood screw with coarse threads to remove what was left of the insert.

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Step 4. Remove the check ball. As noted above, these are on Power valve Protect models. This ball prevents blowing your power valve in the case of a back fire.

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5.

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Step 6. Drill the baseplate. Make sure you drill the baseplate low enough so it doesn't interfere with the metering block. This is very important! I chose to use a 3/16" brake line as a vacuum tube. I purchased it at NAPA for a few bucks and it already has a flare to help hold the vacuum line on. I pre drilled a pilot hole before drilling the 3/16" hole. I drilled on an angle as seen to miss the screw hole.

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Step 7. Plugging the power valve port at the bottom of the baseplate. I found an old lead shot shell for my donor. I punched one lead shot down into the hole and swaged it into the port blocking the bottom of the hole. I added one more on top of that to plug a hole that was made when they machined the baseplate( when they drilled the screw hole they got into the power valve port).

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8. Another view of the swaged lead shot protruding from the hole.

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Step 9. 3/16" tube inserted. Apply two part epoxy on to tube then tap in. Blow into new vacuum line to clear epoxy from port. Any excess will blow out of port.

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Now it is possible for me to run my power valve off of manifold vacuum.
I hope someone can benefit from this article. I only wish I had found something like this before I started all my research (head banging and many unanswered questions). Thanks FordMuscle for this website and allowing all of us to share. Now go bang some gears!
 

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Hey, that looks just like my engine before I dropped it in! Good write up on the power valve referencing. I will also add that on an engine w/ a blower adding jet extensions on the secondaries if you are using a double pumper also helps to keep the motor fed when you go into boost :) -Mike
 

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I gotta say that you did a really nice write up and the picture quality is 2 thumbs up. Thanks.
 

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Thanks for the kudos guys. Sometimes it's hard to put thoughts into words but I did the best that I could. At first it is somewhat difficult to grasp but after researching and understanding what the power valve does and how it operates, it is easier to understand the process of referencing manifold vacuum to the power valve.

Hey Blown 68, sweet engine!
 

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Thanks Blown68, Another item I never knew about. And I know I'll need them with that blower! I just purchased a set of jet extensions and a notched float off e-bay for a total of $24.50 shipped. I love this site!
Hey, that looks just like my engine before I dropped it in! Good write up on the power valve referencing. I will also add that on an engine w/ a blower adding jet extensions on the secondaries if you are using a double pumper also helps to keep the motor fed when you go into boost :) -Mike
 

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Interesting to say the least. This issue was never done at Holley from an engineering aspect. Tons of aftermarket applications but I never saw this done. I'll have to check out their website. I lost a lot of interest in them after they laid us off. I would suspect this could be done to an Edelbrock carb too considering vacuum controls the step-up rods for enrichment.
 
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