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

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



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