Holley Idle Feed Restrictor Mod

While the majority of out-of-tune hot rods on the street are running rich rather than lean, I recently had an occasion where I couldn't get my motor rich enough at idle. No matter how far I turned out the mixture screws, I could not get my 460 below an Air-Fuel Ratio of 14.8:1 at idle. I'm not much of a carburetor expert so when I had my Torino in at the shop, Mike at Mike's Auto Perfection remedied the problem so that I'd have some idle mixture screw tunability with my Holley 670. I recently threw the smaller carburetor on knowing I'd need something milder than my BIGS 950 to pass an upcoming emissions check. On top of that, the price of fuel made the swap an attractive "de-modification". This smaller carburetor yielded the aforementioned idle lean condition in its "out-of-the-box" form. Here's what was done to correct the problem:

Photo 1: Mike opened up the secondary bowl and removed the metering block.



Photo 2: There are a pair of idle feed restrictors in each metering block. To cure the problem, Mike would modify the idle feed restrictors in just the secondary metering block. I've got them indicated here with the red circles.



Photos 3 & 4: With a small drill bit set, Mike determines the size of the existing orifice by inserting progressively larger bits into the orifice until there is some resistance.



Photo 5: A pair of calipers confirm the orifice is .026"



Photo 6: In order to enrichen the idle mixture and to provide the idle mixture screws some range, the orifice would need to be enlarged. Keep in mind that if the idle feed restrictor is enlarged too much, it'll have to be removed and replaced. In that case, Mike increased the size by 7% to .028" being sure to blow out any shavings. It turned out that this was enough to dial in a 14.2:1 Air-Fuel Ratio at idle with the idle mixture screws turned out 2-1/2 turns.



Bonus Holley Tip: Since we had the carb off, we cracked open the primary block too and swapped in a 6.5 power valve. Here's a simple trick to make sure the gasket seats squarely between the power valve and the metering block. Hold the metering block up, seat the gasket on the power valve's groove, and install it as shown. This ensures the gasket will be in perfect register every time.

This is the same problem I had on my 570 street avenger. I could turn the idle screws in to lean the carb at idle , and then turning the screws almost all the way out wouldn't cause difference. Can this procedure be used on all carbs.

Thanks

Greg

Warning, be very carefull about doing mods like this....

There are many ways to solve those lean or rich idle issues.

#1 Timing must be set correctly. Not by the book specs either. Timing is set by your specific engine conditions.

#2 The throttle plate position is extreamly important. If they are set up wrong, it will make you think you need an IFR adjustment, then once it is done, your really in trouble.

#3 Most folks never set the rear throttle plates correctly either. Those are also very crittical to getting a good idle.

This is not a Willy-Nilly adjustment. This is the last resort fix once all other options have been tried. Only someone that really knows carburators and engine tuning well can detemine if this is a fix that will solve your issues.

Looking at the transition slots at the bottom of the carb... If that little square in this photo ( the center-rear of the bore) is anything but a little square, then your throttle plates are not set correctly. Front and Rear must look like this.

Thanks Mikes66. This statement is an important addition to this Tech Exchange Post...

"Only someone that really knows carburetors and engine tuning well can detemine if this is a fix that will solve your issues."

The guy that made this mod to my carb has been dealing with Holley's for more than 30 years. A good portion of his shop's business is tuning "out-of-tune" carbureted street machines and hot rods. I was just happy he was willing to share and let me shoot photos as he quickly came up with a solution for my idle lean condition.

Mike is 100% right, also remember that a minute difference in change in diameter will cause a big difference in fuel. .001 is significant and rarely will you change more than .002.

I have had to drill IFRs but it should be considered a last resort after all the things Mike said

When making changes, you should calculate the area of the passage and make changes based on the change in area (same as calculating the area of a circle A = Pi R Squared (Sorry couldnt get the formula to look right)

Just to add a little to Mike66's post, typically you only want the T-slot showing on all 4 barrels if you have a 4-corner idle carb. If you only have a primary idle adjustment, you'll want the secondary blades sitting right AT the bottom of the T-slot, so that it starts to expose the slot as soon as the blades start opening.

IF you are in a position where to get the correct idle speed, you either have too much or too little T-slot exposed, you can fine tune it by adjusting the secondary blade position slightly (open the secondarys to close the primaries more, or vice versa), or if necessary, drilling holes in the primary throttle blades.

Does Edelbrock have this type of setting on their carburetors. I didn't check prior to installing the 500 cfm on my 289, has a bit of an idle issue .

Greg

Originally Posted by foleysfriend

Does Edelbrock have this type of setting on their carburetors. I didn't check prior to installing the 500 cfm on my 289, has a bit of an idle issue .

Greg

If you need to do that type of work on your type of carb, then you have the wrong carb. Edlebrocks are for no hassel easy and dependable installations. If you want a performance carb, you need a holley based carb.

Jon, Feel free to take my comments and incorporate them into your original postiing. A list of diclaimers before your atricle would make good sense. I am sure you don't want folks doing these mods, just because they now know it is possible to do so....

Motorhead also made a few good points. Without an equal 4 corner Idle cicuit, sometimes it is best to have the rear transistion slots just barely covered by the rear throttle plates. But in some engines where Idle is too rich, then exposing the rear slots is a good way to lean it out a little, as long as the rears are not opened too much.

Also, I would like to add one more situation where this mod is the best fix for some users.

It is for part throttle driving, and low speed cruising. When driving at a constant RPM, and speed ( near 15 - 25 MPH), some folks will get a constant lean stumble, where mis-fires and bumbling along is always present.

This happens when your not quite at idle, and not quite usinig the main Booster Circuits. It is when your engine is running solely on the transistion slot circuits. The transition slot circuit also runs off of the IFR circuit. When this condition occurs, it is a great fix to increase the diameter of the IFR but 2 - 3 sizes (example .030 IFR to .033 IFR) to cure this part throttle lean.

But this does effect the idle circuit as well, and that means you need to re-turn your idling for less idle mixture.

Originally Posted by Mikes66

It is for part throttle driving, and low speed cruising. When driving at a constant RPM, and speed ( near 15 - 25 MPH), some folks will get a constant lean stumble, where mis-fires and bumbling along is always present.

This happens when your not quite at idle, and not quite usinig the main Booster Circuits. It is when your engine is running solely on the transistion slot circuits. The transition slot circuit also runs off of the IFR circuit. When this condition occurs, it is a great fix to increase the diameter of the IFR but 2 - 3 sizes (example .030 IFR to .033 IFR) to cure this part throttle lean.

But this does effect the idle circuit as well, and that means you need to re-turn your idling for less idle mixture.

i've had to deal with an engine like that.
you could watch it on the AFR gauge we use. very slighty throttle opening and the gauge would climb to to 16.5:1 and POP and the engine would stall.
2 thou on the IFR and the problem was solved. though it did mean taking half a turn from each idle mixture screw. (two corner idle, 1850)

though this was a last resort. as we'd had similar problems with the same modle of car caused by the ignition system so we went down that path first