Holley Idle Circuit Mods
#1
If anyone subscribes to Hemmings Muscle Machines there is an article in the December 2015 issue titled "Idle Work" starting on page 64.

It outlines the steps to relocate the Idle Feed Restrictors (IFR) in the metering block(s) on Holley carburetors from the bottom to the top of the idle feed passage thus creating a more responsive idle and circuit performance.

The Chevy guy across the street happen to be throwing out a Holley carb he couldn't get to run on his Corvette so asked if I could have it. After tearing it down it was pretty obvious why it didn't run on his car. The rebuild kit should be here this week so I can start putting it back together but now reading this article I would be interested in trying to relocate the IFR's on this carb.

Has anyone done this before and if so, is there any advice you could provide me before I drill and tap the metering block to relocate the IFR's?

Thank you
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#2
First, before throwing money at it what is the list number on that carb?

I've drilled passages to add adjustable idle circuits on the secondary metering block giving the carb 4-corner adjustable idle circuits. I've drilled and replaced the press in air bleeds in the main body. I haven't seen but understand the mod you are talking about. But I wouldn't try it unless you have new restrictors to install in the upper location.

If the carb is in overall good condition, in my opinion your best option is to install aftermarket billet aluminum metering blocks like those from Quick Fuel Technology and convert the carb to 4-corner idle. The QFT metering blocks have screw in restrictors for the idle, power valve, and emulsion bleeds. They also rework the emulsion bleeds. By the way, the QFT metering blocks locate the idle restrictors in the upper locations. In the end, the carb has a much crisper throttle response just from a metering block change.

Best Regards,
Mike
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#3
Hi Mike,

Thank you for responding. I enjoy reading your responses and appreciate your technical knowledge here on the site. I do not have the list number in front me at the moment but it is only a 600cfm carb, nothing spectacular or rare by any means. I will provide the list number once I go to my garage.

The carb is in good condition but just needed to be thoroughly cleaned and have a rebuild kit installed. All the mechanicals appear to be working fine.

The article does talk about achieving this mod by simply purchasing the aftermarket metering blocks which already have these modifications. I think it would be interesting to try to do the mods myself following the steps in the magazine.

The new IFR's are actually brass set screws which need to be drilled to the size of the original IFR orifice then tapped into the upper location on the metering block. The article also provides the sizes of the set screws needed as well as where to purchase them. As the article discusses you can then experiment with different size IFR orifices simply by unscrewing the IFR and replacing it with another size.

It looks fairly straight forward in magazine plus if I screw it up then I can always purchase a new QFT billet style metering block you mentioned above. After all, I have nothing to lose (maybe a few bucks) and only "know how" to gain. Currently I am running a 600 Edelebrock on my car but would be interested in trying this carb out to see how it performs.

I will try and post the article so others can see it.

Mike
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#4
This is an interesting discussion to follow.

Does the article mention why Holley does not build their carburetors with such a modification in the first place?

I will be interested to see what how the actual work of the modification gets on and then of what you think as comparison between the current Edelbrock and the Holley performance.
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#5
Sounds like it may be either a 600 CFM vacuum secondary, List 1850, or mechanical secondary (Double Pumper), List 4776, carb. Let us know what you managed to get for free.

I thought of doing that mod many years ago simply so I can have screw in IFR's and more idle A/F mixture adjustment. I tried it on an old metering block I have. At least on mine the upper IFR location was already the correct size and needed only to be tapped. I went with QFT billet metering blocks and didn't follow through with the mod. I was fearful it wouldn't work and some of Holley's metering blocks are still hard to get.

You can purchase screw in idle feed restrictors from Summit Racing. they might be the size of the set screw the article specifies. If so, you can get an assortment of a few different sizes.

You have to spend a lot of money on a Holley Ultra HP series carb to get metering blocks with screw in IFR's in the upper location. All others are press in style at the lower location including the Ultra Double Pumper series with billet metering blocks. Why, I don't know. QFT carbs are gaining popularity probably because they include many small additional features like this one.

Best Regards,
Mike
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#6
Hey Mike,

I needed to cross reference the part number on the carb to get the model # on Holley's site. The carb is a 4160, 600CFM. It has an electric choke with vacuum secondaries.

That is great that Summit carries the idle feed restrictors. The article references the correct size set screw as 6-32 x 3/16 brass set screw that can be purchased at Fastenal.com, P/N 0151356, pack of 20 for $14.

Also indicates HFS steel measuring pin kit 0.011 - 0.060 in (for measuring the correct orifice size) can be purchased on Amazon.com P/N 15984 for $16.

Summit has the Quick Fuel primary metering block P/N 34-8 for $65 and the secondary (if needed) for $67.

So, I can expect to spend at least $30 plus shipping to do this myself or save the metering block and purchase it from Summit for $65.

As always, I appreciate your help!

Mike
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#7
(11-04-2015, 12:54 AM)Steven Harris Wrote: This is an interesting discussion to follow.

Does the article mention why Holley does not build their carburetors with such a modification in the first place?

I will be interested to see what how the actual work of the modification gets on and then of what you think as comparison between the current Edelbrock and the Holley performance.

Steven,

Good question and yes they do mention a reason. Holley does not build their carburetors with this modification because they do not know the application the carb is going to be installed on. They use the example of the 750 CFM carb whether it is going to be installed on a near stock 283 or 650 HP 496 cu. inch big block (sorry for the Chevy references but I am sticking to the article)

Paraphrasing the article:

Holley manufactures universal type carburetors by design suitable for a wide range of engines.

Hopefully this answers your question and I will keep you posted once I complete the modification. I am thinking it won't be done before Thanksgiving due to my heavy schedule at work.
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#8
Measuring Pin kit...I learn something new all the time. I've using my number drills. They seem to work but I am occasionally between sizes.

I find it interesting the article mentions "Holley manufactures universal type carburetors by design suitable for a wide range of engines". Most people bolt one on and are quickly discouraged by some drivability issue or small lack of performance. And not understanding they are universal to some degree and tuning is very often needed to optimize the performance and obtain good drivability.

Best Regards,
Mike
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#9
(11-04-2015, 11:34 AM)1969_Mach1 Wrote: I find it interesting the article mentions "Holley manufactures universal type carburetors by design suitable for a wide range of engines". Most people bolt one on and are quickly discouraged by some drivability issue or small lack of performance. And not understanding they are universal to some degree and tuning is very often needed to optimize the performance and obtain good drivability.

Best Regards,
Mike

Mike...you are right on. Quoting the article:

"While the average enthusiast often expects that once he bolts the carburetor on his engine, it will instantly start, idle and run well, it's likely the idle circuit will be intentionally rich to accommodate the wide range of different applications."

So you are correct as noted above..there is some degree of tuning required for optimal performance.
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#10
This is all very interesting to me. I am old school SU and Webber on Euro engines so only have minor experience with the Holly or engines over 3 ltrs.
Her M1 has a Holly so I may have to learn more very quickly. I am still having most trouble with idle at all engine temps. This info could well help.
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