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School me on what IFR's are? ,and why should I wire them down? In your earlier post, you mentioned PVCR's also? Power Valve Circuit something? Still learning these acronyms. Thanks, Jay

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IFRs are the Idle Fuel Restrictors. One for each barrel. They meter the fuel to the idle screws. The Idle Air Bleeds in the throat of the carb meter the air. Together, they provide the ratio for the idle mixture. The adjustment screws are simply a volumn adjustment.

If your adjustment screws are only 3/4 turns or so out, then the mix is usually too rich. If so, then you can either size the IFRs down or you can open up the IABs. I like to size the IFRs down first, that way I don't mess with the timing of the main circuit.

The proper way to adjust the jetting on the IFRs is to drill and tap them for jets. But what I do is to mike a bunch of strands from some 12 to 16 guage electrical wire and use pieces of that inserted in the IFRs to see what the engine likes. Quick, easy, cheap and effective.

I leave them in when I get them about right, but a guy could go back and drill and tap them for the jets.

PVCRs are the Power Valve Channel Restrictors. They are the holes under the power valve and are essentially jets. The main jets plus the PVCRs add up to the total amount of fuel being fed. Which is why you step the mains up when you block off the power valve. The mains must be larger to compensate for the PVCRs no longer flowing any fuel.

The PVCRs can be drilled larger if the main jets are stepped down or can be wired down if they are too larger and providing too much fuel. Either way, the mix overall (mains + PVCRs) usually needs to remain about the same.

Holleys, in my experience, are usually close at Wide Open Throttle. But need a lot of help in the low speeds, transitions and cruise. Which is where a street car spends most of it's time. There are many, many combinations in these circuits on any given size carb. Which is why all carb manufacturers make street and track versions of the same size carb. It is not just about mechanical or vacuum secondaries.

Track oriented carbs like the DPs, are much richer in these areas than a street carb needs to be. For one thing, a track carb isn't too concerned about cruise.

Do you have Emanuel's Holley book? You will need it. And if you are going to experiment with the IFRs, IABs, and PVCRs, you will need to get familiar with Pi x r2. That gives you the area of the holes you will be modifying. So you know how much bigger or smaller you have made the flow.

This may be the longest post I have ever made. It had better help!

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IFRs are the Idle Fuel Restrictors. One for each barrel. They meter the fuel to the idle screws.

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Not quite, They meter fuel to the transission slots.
The transission slots are the discharge ports for the intermediate circuit.

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School me on what IFR's are? ,and why should I wire them down? In your earlier post, you mentioned PVCR's also? Power Valve Circuit something? Still learning these acronyms. Thanks, Jay

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You dont have to wire them down as DaveRS23 explained what you do is insert a piece of wire a certain size in the IFR (Idle Fuel Restricter) which takes up space and lets less fuel get through that circuit and leans out the idle. Its an old trick Bill Jenkins use to do back in the 60's and 70's and it works. All the newer cars already have jets in the IFR and the PVCR and on the newer carbs all you have to do is change the jets. In fact most newer carbs have jets in the ildle air bleed and high speed air bleed circuits also for easy tunning. Ron

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IFRs are the Idle Fuel Restrictors. One for each barrel. They meter the fuel to the idle screws.

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Not quite, They meter fuel to the transission slots.
The transission slots are the discharge ports for the intermediate circuit.

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IFRs are the Idle Fuel Restrictors. One for each barrel. They meter the fuel to the idle screws.

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Not quite, They meter fuel to the transission slots.
The transission slots are the discharge ports for the intermediate circuit.

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Well...

The slow air bleeds also called the Idle Air Bleeds combined with the Idle Fuel Restrictions work together to create an emulsion or fuel/air milkshake. The richness or lean-ness of this mixture is determined by the ratio of these air and gas jets.

This emulsion is delivered in whatever quantity your motor can suck from the transfer slots at low throttle openings before the mains come on. The same emulsion comes from the idle discharge holes (sorry don't know the fancy acronym for that one). The corner idle screws, often called idle mixture screws by some, cannot change the a/f ratio of the emulsion. They do not mix anything they just act like a valve on a garden hose- you should be able to turn them off and kill the motor.

Looking at your carb from below the butterflies at idle you'll see the idle discharge holes and if the curb idle is set correctly you'll see around .030 or a square section on the transfer slot. If you show too much slot below the butterfly at idle it will overwhelm your idle discharge fuel and render the corner 'mixture' screws useless because the motor will suck on the t-slot for idle. Thats why sometimes you need to open the secondaries a hair at idle too.

To a certain extent the circuits all stack up- idle, transfer slot, main jet, and on low vacuum the power valve. Thats why you should get your timing right, then your idle 'mix' screws for max vacuum, then tune the T-slot for clean low throttle cruise, then go back and reset the idle screws, see how low you can get the main jets and drive like an old lady, then get the power valve to help out when you need a little more power, then tune the squirters and secondary mains for tire shredding.

This is a rare thread when I was looking for this info I got kinda vague responses and had to spend a lot of nights pouring over holley books and internet misinformation. Once I figured a few things out folks seemed to act like it was common knowledge! "Of course you have to restrict the IFRs on a street car with a holley DP"

You seem to be asking good questions and getting great answers. Please use this info as a jumping off point for your tuning- get in there and try some stuff and come back and let us know how it worked for you. Then later other folks can search and find this stuff out too

Radar

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By the way that was not meant to snub the fella that described the t-slots as the 'intermediate circuit'. He is right it does flow fuel after the idle and before the mains.

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Understood. I didn't want to get "into the weeds", but just to somewhat clarify a little.