Holley idle air bleed tuning with wire....need some experience.

[cleandan]

I am trying to tune a Holley carb that is on top of a 1967 427 FE in an original Cobra roadster.
The carb info:
LIST 3255
494 date code
C5AF-9510-BE

The (BE) variant is making it difficult to find specific info for this carb.
I can find all kinds of info for a few other similar carbs, but not for the (BE)

Anyway, I have it running pretty nicely, but it is lean surging at light throttle, slow speed cruise.

The idle air bleeds are very large, and of the little info I have found on this carb, some of it talks about this carb having a lean idle from the factory.

I really don't want to damage this carb as it is rare and valuable because it is the carb this Cobra was built with back in 1967 when Shelby made the thing.

I am wondering about richening up the idle air system by installing wire in the bleed hole.
I figure this will get the carb tuned, while not altering any of it or causing damage.

I have not done this before and wonder what is the best method of affixing the wire into the bleed hole?
I am certain others have done this mod before so some experience help would be great.

While I have not yet measured the current bleed hole, it is large, probably in the .070" range.
If I make the hole roughly 1/2 its current size how much will that effect the bleed mixture?

I know I will be trial and error with this tuning, I just want to know the best method for adding the wire restrictor for the most permanent, yet reversible, application.

Thanks and have a great day.

[My427stang]

Changing the IAB will make it idle fuel come in more aggressively but won’t change the a/f significantly

If it’s lean, I would start by raising the float level. It won’t do a lot but sometimes you can see if you were on the right track. Additionally another quarter to half turn on the idle air fuel screws should richen it up a little bit. Last and more effective would be to add some IFR fuel probably .002 increase would be noticeable

When you are running on the idle circuit, actually for all circuits, the air comes from around the throttle blades, Air bleeds affect circuit timing and provide air to emulsify the fuel to allow it to be atomized easily through whatever orifice it enters. Air bleeds do not really add or subtract a/f

However to answer your question closing it down by 50% is quite a lot if you calculate the area of the hole you can do a 2-5% change rather than a huge change

[Falcon67]

Not the sharpest carb guy, but agree - at light throttle/slow cruise you're running on idle + intermediate fuel so the IFR would be the place to tinker, not necessarily the IAB.  If the published diagrams are correct, the IFR controls fuel from the main jet into the idle + intermediate circuits.

[Thumperbird]

I would say you most definitely change the A/F, ok maybe not "significantly", just the fact that the curve changes is proof of that for a given rpm.  I can not speak to your specific model but I have done a fair bit of tinkering on a 2x4 setup and the IAB definitely impacts ratio at idle and light accel..  I played with wires, they do work, can not speak to a permanent mount setup though, I'm sure one of the carb guru's will chime in here. Might not be ideal but for some minor trimming it beats the heck out of cracking things open some times.

[cleandan]

I am looking to make some changes that will prove a more permanent alteration is in order.
I do not want to alter the IFR without knowing this IS the change this carb needs.
Trying a few different wires stuffed into the huge idle bleeds seems like an easy, non-permanent, test.
I just don't know what to look out for when doing the wire test.
wire too long.
wire gets sucked into carb.
wire vibrates deep into metering plate orifice.

Knowing what not to do can help me get a better result I hope.

[Falcon67]

Personally have never throught about using wire in the IAB, not sure how you'd hold it in place and it'd have to be whisker thin.  Only procedure I've seen discussed at length is to remove the IAB and insert another, or a brass plug that has been drilled.  I have used small pieces of wire in the IFRs on a Holley to tinker with idle fuel flow on stock metering blocks - those are easy as an L bend lets them be trapped by the metering block to body gasket.

Since this is one of those high buck hard to find carbs, I'd think strongly about finding a close relative and hacking on that one to find the parameters for the IAB and IFR, then consider what may need to be done to the stocker to fix the issue.  And then decide if it's worth it to modify a rare unit. 

[Cyclone03]

Sadly the old Innovate message forum got nuked. (Innovate O2 indicators) .
10 years  ago a poster with the handle “tuner” had excellent insight on tuning Holley and Holley based carbs. He included some theory as well as practical application.
The cool thing was his insight help one “see” the function of each circuit and how the overlap,transition,one to the next effected driveability.

I also learned the whole carb too big on the street thing is sort of a myth ,but a small carb is easier to tune on the street because there is a lot less to explain. Idle is idle,main is main jet, power is ….well main jet.

Now a big carb it’s IFR,IAB,idle mixture screws,Main Jet MAB,PVCR,PV,secondary …..you get the idea.

At the time the 850 Demon came on a brand X crate motor,and everybody that cared tossed them because they sucked driving around town. I got mine for $50.
I ended up with every hole drilled and threaded for replaceable jets,IAB,MAB,PVCR,IFR .
I tuned that thing to 18mpg in my FE Mustang and 12.0s in the quarter.
It ran great.

I also learned that back in the olden days hot cars came from the factory.
The best OEM tuned Holleys are the last ones fitted from the factory. Look at the ‘vette 3310 or the CJ 735,later the 351c carb. You will find excellent low throttle angle tuning and driveability in those carbs.

Three modern tools will teach ,if you pay attention ,which area to tune. O2 indicator,vacuum gauge and tach.
WOT can still be done with a drag strip and reading plugs but drivability can be nailed with feel and tools.

[338Raptor]

I tried the wire in the air bleeds technique.   I don’t recall the wire diameter I used but I know it was very small and it richened up the mixture way more than I needed.  To make small adjustments I doubt you can find a small enough wire.  You may need to order a new smaller bleeds than you have now (assuming yours are replaceable). 

Do the math.  2 smaller size bleeds is an area smaller than any wire you will find. 

[1964Fastback]

You guys are probably aware of this, but one way to get small wire is to take regular multi-strand speaker/power cord wire, strip the insulation, and just unwrap individual strands.  As always, use at your own risk.  Not sure if this technique makes the wires more brittle or prone to breaking.

Pat

[GerryP]

You can't use just any small wire.  They are different sizes.  After doing the math for my particular application, I did strip various wires and found all of the individual strands were too thick.  I found that a wire clipped from a parts scrub brush was smaller than strand wires and the right diameter for what I needed.  I also read that modeling telephone pole wires us thinner in diameter.  At any rate, you have to do the math to see what size wire will reduce the area for what you're trying to achieve.  And I would make sure you are dealing with fuel issue that can be solved only by fiddling with bleeds and restrictors.  As was mentioned above, raise your float level and turn out the mixture screws to get you in the zone.  Also, make sure the carburetor is really clean.  Ultrasonic clean it until you know for sure all the metering passages are clean and free of any deposits.  A spec carb shouldn't have that issue.

[cleandan]

I have been fooling around with the carb and here is what I have so far.
The primary idle air bleed hole measures .076".
1st wire was .016". It made a noticeable change, but not enough.
2nd wire was .020". It made more positive change, but not enough.
3rd, and current wire, is .026". This wire is very close, but needs some specific tuning.

The wires are carefully bent into a sheppards hook shape and pushed into the bleed hole and have some friction over the hole/carb body to keep them in place.
I also bend a curve in the long portion of the hook to give it some friction in the hole.

I had previously tuned the idle mixture to achieve highest vacuum (9.5-10")
I have not changed the idle mixture over this whole wire test procedure in order to keep all changes due to the wire.

The .026" wire has greatly improved the lean cruise issue, it is almost gone.
Now I will mess with the idle mixture screws to see if it will get even better.

Attaining small gauge brass, copper, or stainless steel wire is not difficult at all.
I happen to have many variations of each due to my work in watch and clock making.

Anyway, for the small effort in carefully making the wire hooks, and the time taken to install, drive, test and tune, I have made positive progress and proved my hunch about where this carb needs some special tuning.
Whether that special alteration takes place is still to be determined because this is a very rare, short production run, stupidly valuable carb.

As always, I greatly appreciate all the helpful replies...They do make a difference.
Keep the ideas coming.

[e philpott]

Chevy 396-375 and 427-425 had Holley with adjustable idle air bleeds , I had a 7 of those carbs given to me from a super stock racer of the late 60's and they were all good carbs , I could tune in the bleed sitting there idling , it didn't care if it was a 454 jet boat with strip dominator or a 306 SBF Shelby cross ram with single 4 top , awesome carbs and the bleed size made a big difference on those Carbs for the idle circuit ... I'm down to only one of those carbs now

[Drew Pojedinec]

Ok.....  first off, 99% of the lean issue with Ford carbs has to do with the IFR having some trash in it.  Unlike later carbs, the IFR is located IN the idle well.  you have to remove the plug, clean the ifr and replace with new plug.
Due to it's location, it's pretty common for trash to flow back and forth creating intermittent issues.

Trying to change air bleed size generally has a fairly small impact.  Changing ifr even slightly has a much more dramatic impact.
That said, unless you  have a 240@.050 + duration cam, you shouldn't need to make any changes.  Yes it's a fuzz on the lean side, but an adjustment to the mix screws should deal with it.

Personally, if you do have a large cam, I think drilling ifr to .033-.035 is the answer here... it's a small change, but in regard to idle, richer is easier.
As far as altering the carb, honestly, any "real" carb builder  wouldn't care as it is easy enough to reverse the changes.

For what it's worth, a 3255 is not a hugely valuable carb, heck I have 6 of them.  the Date on yours is very very early, and you should certainly take that into valuation.  Rarely do you see one from 1964.

Down to the meat and taters.... here is the typical calibration:

List number         C5AF-9510-BE
Type            4150
Date            581
Primary            
Float            Lemans Bowl
Booster            .140 Downleg
Pump nozzle         .025
Pump type and cam      30cc White #1
Idle air bleed         .075
High speed bleed      .028
Angle Channel         .139
Metering block#         4137
Main Jet         72 (stock is 76)
Power valve         6.5
PVCR            .066
Emulsion         2 @ .027
Kill Bleed         none, emulsion tube feed .025
Idle Feed restriction      .031
Mixture screw hole      .063
Needle and seat         Lemans bowl
Venturi size         1.375-1.4375  1 3/8-1 7/16
Throttle plate size      172/172
            
Secondary         
Mechanical or Vacuum?      VS
Spring color         long yellow
Diapghram length      short
Float            Lemans bowl
Booster            .140 downleg
Idle air bleed         .028
High speed bleed      .025
Metering block/plate#      4241
Emulsion         2 @ .028
Idle Feed Restriction      .031
Jet size         85
Needle and seat         Lemans bowl

[Falcon67]

You guys are probably aware of this, but one way to get small wire is to take regular multi-strand speaker/power cord wire, strip the insulation, and just unwrap individual strands.  As always, use at your own risk.  Not sure if this technique makes the wires more brittle or prone to breaking.

Pat

When messing with the IFR in the metering block, I typically used 16 or 18 gauge stranded copper.