Calculating IFR/air bleed size on a Holley (long)

[My427stang]

Does anyone have a formula before I start drilling? or throwing carburetors LOL

Or for that matter, do you have another direction you think I should go?

I have very low speed (1000-1500) surge when at little to no throttle. More like a buck than a surge. Very annoying in traffic. Once you get into the accel pump or more throttle to get into the main circuit, its a downright monster and doesnt miss a beat

This problem has been following the motor through 3 builds, the carb is 3 yrs old or so and was purchased new, all Holley parts inside, etc.

1st time I blamed a bad intake gasket and found a bad intake manifold surface which we machined, thought it was just a vacuum leak

Still there, chased it for a long time with basic tuning, but cam was big with a big single plane intake, and only 7 in of vac, so blamed the cam, intake combo with only 427 cid and just ran it, figured it was classing overcamming, big plenum stuff. Its not severe, but its always there

Now, fresh 489 cid motor, 13 in vacuum at idle, dual plane intake, 1000 HP series Holley vac sec (dimensionally an 850), less cam, timing has no real effect

As I thought about it, this hiccup has followed the carb through all the builds, its not a severe as a cold no-choke kind of burp if you old hand choke guys know what I am saying, but its the same feeling. At a light 1000-1500 rpm cruise through a parking lot it'll buck and lurch just enough that it takes up the slack in the driveline and sound horrible, so you drop a gear and bring it up 500 rpm and its fine. But its very annoying.

I thought I was going to find a lean idle circuit due to a small IFR, but when I opened the carb up its got .045 IFRs, pretty good size, I didnt expect it, I was going to drill bigger if they were under .035, but it seems to me I should have enough fuel with that big of an IFR

.067 air bleeds, but I have run up to .076 as experiment, no change.

Is there a mathematical way to calculate a starting point for IFR / idle air bleed relationship based on venturi size, cid, vacuum, or some other variable?

I am currently at 13 in vac, at 14 degrees initial timing, all 4 idle screws idle best at 3/4 turn out, and all 4 will kill the motor. Float level does not fix the surge but may have helped a little, went as high as I could until it bubbled out the boosters. Thats why I thought it needs more fuel

Specifics below, thanks in advance (again it's followed the carb, so I dont think its an engine problem, only the carb and distributor are the same in this build)

Engine
- Center oiler - 4.277 bore
- SCAT 4.25 Crank, SCAT 6.70 H-beam rods
- Diamond 17 cc reverse dome pistons, polished and shaped, between .007-.009 in the hole, .005 piston to wall clearance
- CJ windage tray / Canton trap door T pan / Melling HV pump
- KC Stage 1 heads, 2.15 / 1.67, reworked by KC again by hand (Stage 1.5? LOL), chambers turned out to be true 73 cc
- Edelbrock RPM intake, pushrod pinch opened to 1247 gasket, plenum divider notched 2" x .5" , matching 1" 4-hole spacer opened to match shape of plenum (doesnt look like a 4 hole now, looks like 2 ovals with center divider notch
- Erson rockers
- Erson Cam 286/294 242/246 .595 .595 110 LSA degreed to 110 C/L, Comp lifters
- Worked out to be 10.41 static / 8.01 DCR by the numbers
- 1000 cfm Holley HP series vacuum secondary, BG billet base plate
- Mallory Unilite 14 initial, 24 mechanical in by 2800, ported vac advance (tightened up to only add 4 degrees more as an experiment)
- Hooker Super comps, ceramic coated, 3 inch exhaust to the back of the mufflers, 3 inch H pipe, 3 inch 3 chamber Flowmaster Deltaflow mufflers, 2.5 inch mandrel bent Flowmaster tailpipes

Tranny/clutch/rear
TKO-600 2.87 1st gear / .64 OD
McLeod dual 12 inch friction clutch, diaphragm
3.70 9 inch, Trac Lock (Currie clutch cover, clutches, Moser axles, underride traction bars

[ClassicComp]

I would reduce the size of your idle airbleeds by .006/.007 if your IFR's are .045.
How much ignition timing and vacuum do you have at that rpm?
to calculate area is radi squared x Pi
or .067x.5=.0335 radius .0335x.0335=.001122
.001122x3.147=.0035309 area

[My427stang]

I'll check timing tomorrow AM at 1000-1500, work is getting in the way LOL

Man .006/.007 is a huge amount, I already went from .078 to .067 thoughts behind going more? Seems like I have a bunch of response already, if I go .060 almost seems like it'd be a solid shot of fuel (maybe thats your intent)

Thanks, just asking to understand why, not questioning knowledge. I'll get full timing curve mapped out ASAP

[Stevespeed]

Opening your idle bleeds from .067 to .076 leaned / delayed your idle fuel not richened. Go the other direction to .060 and try it. Why are you trying to lug / loaf around with your hotrod at 1000 rpm anyway? Your lifters will thank you for just a little more rpm to help out with oiling.

[My427stang]

I'll give it a shot

However, as you can see, its not a high rpm build. I am not lugging it around though, I just cant hold a steady rpm outside of the main circuit.

It makes for a car you dont want to drive, even across a parking lot

[My427stang]

I sort of misrepresented my first post

I didnt open them to try to make it better. They were .070 stock, and I went .076 originally thinking they were like a jet, vice an emulsifier type air bleed because of a rich (smelling idle) Foolish beginner stuff

Now I am at .067, but the surge remains. I'll try a wire in the .067's and see if it gets the fuel moving quicker. Thanks again

[Barry_R]

Them darn FEs...

He's giving you good advice on the idle bleeds. On a hot piece with a good size carb they might be rather insensitive - - taking a larger change than you'd expect to give results. I can throw a couple other thoughts out for whatever they're worth.

You mention a baseplate change. Was it just the base plate or the full throttle assembly? If the throttle plates are not perfectly centered up in the bores you will have a variance in closed throttle airflow from bore to bore, making tuning tough at low speeds. One venturi will read rich while the other reads lean... If the carb comes off hold it up against a light and sight the blades - if any light at all you should see a thin clean ring of light around all of them.

At low speed/light load more timing can really help. Have you tried putting the vacuum advance into a manifold source? I used the programmable functions of my FAST system to do essentially the same thing - - I crank/start at 20, carry a lot of lead(30+) at low RPM/light load, and back it off progressively to catch the 28 degrees the engine seems to like at WOT. A backwards timing curve...

A lean idle condition will also smell. The sparse mixture won't light off consistantly - sending unfired fuel into the exhaust. It'll send ya chasing yourself sometimes - - I know that from personal experience.

[My427stang]

Thanks Barry

I did try more initial, but my curve was so quick it really rattled, so if it needs some I need to slow the curve a bit. It may have helped a little, but not much

I'll give the bleeds a shot, sure would be nice if it worked.

The baseplate was a change to a BG Idle ease because some monkey (me) broke his second baseplate in 20 yrs at a 20 yr interval. It was a full assembly, but I checked positioning last night just the same and it was very very close to perfect. (BTW I dont use the idle ease, it just confusing things more, I just wanted the billet plate)

I also checked transition and idle circuit passages to make sure they lined up and they are spot on now, were off a little going through the gasket, but it didnt change anything

[MadBill]

I presume you've checked that there's enough transition slot showing under the blades at idle, say 0.040" or so? and on the secondary blades also?

Before changing any more jets/restrictors, try slowly cranking up the idle with the idle speed screw (what RPM is it now, BTW?) until it starts running rough, then twiddle the mixture screws to see what it wants. (And the IFRs make a lot more difference than the air bleeds.)

If all else fails, you can get pretty good at dipping the clutch and starting over, otherwise more throttle often just gives worse surging...

[My427stang]

The odd thing is at no-load I cant get it to run rough and buck like it does on the road.

My idle right now is about 950. The buck is anywhere from 1100-1600 -ish under very light load

There isnt a lot of transition slot showing, I played with closing the rear plates and opening the front just to do that, maybe have at least the front just a bit deeper into the slot to get it pulling a little quicker.

I tried it, and the front adjusters seemed to have more authority and the backs were much worse. Unfortunately, I dont think I can bring the idle down if I get both front and back open .040. Maybe you are on to something, where else could the air be coming from? A 4 barrel with 4 1/34 inch blades shouldnt be too big for this motor.

I didnt get it out on the street last night, but I dont think its going to do a darn thing, I can still hear that its working to idle. It just seems like this carb doesnt seem to like to idle/transition nicely for lack of a better term.

I almost have to blame the carb, with so many intake changes and different heads and cams, etc over the course of the past few years, the only common parts are the carb and the distributor, and more than that, every single part barring the better flowing heads, promoted more vacuum / better idle than the old high rpm build.

I did check the metering plate surfaces, flat with a straight edge but I didnt mill them, gasket marks on the old gasket look perfect. Base plate to carb spacer gaskets seem fine, but look a little wet, like its trying to wick fuel, but it doesnt get anywhere near the edge of the gasket. When I found a 1/2 hole baseplate transition hole covered by a gasket I was sure I found the issue, didnt change a thing.

I followed every vacuum and fuel passage in the carb. Every one is clear, the only thing I dont really like is that the BG baseplate PV channel has blowout protection and BG's solution to that really chokes down the port compared to the Holley. However, my problem is well below PV load and RPM, so I dont think it is the issue, mine runs good at a higher RPM roll on.

Here some specific examples, maybe it'll help describe it a bit better.

- Timing has no effect

- No matter how I set this thing up, at 900 rpm, if I turn the steering wheel, the load of the PS pump sometimes wants to stall the motor (not a PS pump problem LOL, like a no torque problem)

- When I adjust the 4 idle mixture screws, I turn in very slowly and it blubbers a little but not much, but then all of a sudden, it just shuts the car down, can hardly catch it sometimes. Then as I turn it back out, normally a Holley "cleans up" a little bit at a time as you get to the right spot, this one doesnt really, although they do affect idle, they dont do it in a nice "back it out until it idles best" kind of way, its sorta like a soft adjustment, not much real effect as you back it out, just idles or kills it (sort of an exaggeration, at 3/4 out its clearly idling the best, but not as significant as most others I have seen)

- 900 rpm sounds like a lumpy big cam idle, 1000 smooths up, under 900 it can barely keep itself running. I know this isnt a stocker, but its a 489 inch dual plane, 286/294 110 LSA cam, it just seems like without a whole lot of overlap, it shouldnt be this ragged.

- Vacuum is stable at about 11, again I expected more, but it did come up from the old motor

- Put a .020 wire in the .067 air bleeds last night and I couldnt get it to idle at all, very ragged and ugly. Actually considered opening thes econdary bleeds back to .070 and see what happens

One last description of the surge just to give you guys a better idea of the situation.

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1 - Start the car, if fires right off, lots of snap when you crack the throttle, no choke cold blooded, as expected. Everything below is after warm up.

2 - Let the clutch out does fine, 15 mph area around my house you need to run in first gear and keep it above 2000 or it surges lightly, not horrible buck like its leaping, but enough that you hear it taking up the slack and releasing in the driveline and you cant continue, so I drop a gear. (Did exactly the same with 50 cid less and bigger cam, single plane intake)

3 - Start driving gentle, 2500-3000 rpm shifts, doesnt like to recover from the shift, old truckers know what I mean, you cant shift it "pretty" it'll buck/hiccup a little at the next gear

4 - Full, 1/2, or 3/4 throttle, the motor screams, no flutter, no surge, no breakdown, pulls like a freaking freight train. Its just that idle / transition circuit between low rpm shifts and 5th gear or parking lot running.

I know someone is going to chime in that its a hot rod, but it just isnt right.

489 cubes with not much duration,wide LSA, a dual plane intake, and not a lot of port size seems like it should be pretty happy on the street, and it isnt. Matter of fact, if you arent beating it, all it does is piss you off because what seems to be a mixture issue. LOL

[bill jones]

-The very first thing that I thought about when you first mentioned the bucking problem----was a lightweight flywheel.
-Then you stated that you have a 12" dual disc????.
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-The only time I have had this bucking issue is with a lightweight flywheel and clutch---and with clutch discs that the torsional springs in the clutch disc are dead, too soft or nonexistant.
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-From all the details you have listed of the carburetor the only issues I would belabor are:
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1-I would look at carburetor like a 780 vacuum secondary----look at the secondary idle circuitry real close--then see if you can close the secondary clear down off the secondary slot or so where you only have just a few thou showing at idle.
-try to shut off ALL the secondary idle fuel and make the engine idle on the primarys only except for the absolute least amount of idle fuel being introduced into the engine from the secondary side just to keep the secondary fuel in the bowl from going stagnant.
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2-on the primary idle circuit---where are the idle fuel restrictions?---up near the top or down low near the power valve?
-might want to try relocating the ifr's from wherever they are now to the other location.
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3-I have NEVER had to run IFR's at .045" (but I'm at 4200 elevation).
-I had a pair of 468BBC marine engines where I ran two different pairs of the largest hydraulic camshafts that I could buy from Crane----something like 260-266 at .050"----and at the time these were the largest lobes I could find to buy.
-It took me a week of working on one engine and the 855cfm vacuum carburetors #3418 but these big nasty engines would idle at 500rpm and not stall when loaded against a mild load on the dyno.
-These engines have to idle into the dock from quite aways out---and not create a wake in the water---so they had to idle down under a light load.
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-I have done it so I know for a fact that Holleys can be made to idle on about the rankest idling engines you can think of.
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-The main thing about it all was getting the blades WAY down on the transfer slots.
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-Personally I have NEVER liked the adjustable secondary idle where the secondary blades area adjusted up onto the transfer slot----and I have found that I can usually fix poor idle problems by converting the adjustable secondary idle back to like what a 780 nonadjustable idle circuit is.
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4-on 4412 500cfm 2barrel carburetors there is a small hole--about .023"----drilled above the transfer slot about 3/16"up into the main body that is drilled back into the transfer slot fuel feed channel.
-This extra hole acts as more air bleed at idle to the slot which leans the engine out further than normal when the blades are closed----but it also is sort of like a third circuit discharge when the blades get open just a little past the top of the slots and dumps a little more fuel up there at part throttle.
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5-you mentioned that your engine pings if you bump the idle timing up at idle.
-My experience would say to shorten up the 24 degrees of mechanical to something like about 14 degrees--then bump the intial timing up to 22 degrees so you still have your 36 total.
-You might have to slow down the advance rate l with a two step spring arrangement so that the last 6 degrees or so doesn't come fully advanced until about 3500.
-Typically I'd run about 10 to 14 degrees of vacuum advance on top of that but the vacuum advance would have to be fully advanced above about 10" and totally gone by about 8" and below---which is somewhat of a trick to do but can be done.
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-I also want to say that I am very impressed with your knowledge of your carburetor.

[My427stang]

Thanks for the kind words Bill....I wont be impressed with myself until I fix it but I do appreciate it, grew up in a shop where you listened because someone may not help you twice if you dont.

Its a steel wheel, plenty heavy and not dual disc, dual material, but standard diaphragm kind of McLeod. Nothing light about it

I think you are onto something with opening the primaries though, matter of fact when you said it, it sort of turned a light on. The secondary transition slot isnt helping me at all and I can use some additional response from the front. Unfortunately heavy rains here, I'll try it tomorrow, close it right down like a 2 idle circuit and focus on opening the front as far as I can without energizing the ported vacuum

The IFRs are as delivered in size and location, seemed huge to me too. Its the HP series met plate with the IFRs up top, not the old style down below

As far as timing, I will happily change the curve where the motor likes it best. Fresh motor, 489 cid, not sure if my starter will spin it over way up there, but it doesnt hurt to try. The Mallory is real easy to set total timing.

As far as your boat analogy, thats what I am shooting for. I dont need 500 no-wake rpm, but I sure would like to see something like that, and with my setup I sure think I can.

I'll crank those secondaries closed just before they bind and put it on all primary idle and we'll see what happens and report back. Thanks again

[My427stang]

Update

I took the carb off to better close up the rears and open the front.

I now have it fully closed just before bind in the rear, and about .040 transition slot showing. Ported vacuum is dead at idle

I still have the surge but it dropped about 200 rpm lower. Its livable, but, the most annoying thing right now is one heck of a buck at deceleration.

Basically as you slow to a light, it'll buck pretty hard. It still bucks 1000-1500 lightly, but I think we are on the right path.

You guys think its time to take .001 out of the primary IFRs or got another idea?

[ClassicComp]

I have a 383 sbc I wrench on that only make 5" of vac @ idle in gear and the ifr's measure .034 w/a .062 air bleed,2 idle mix screws are out 1 turn from seated.
This car drives around @ low speeds without surging or popping.
Cam specs are 250/260 @ .050 w/a 106 lobe sep this will idle @ 600 out of gear if required.

I'm sure you know this but the ifr supplies the mixture volume and the air bleed emulsifiys the fuel mixture also effects where the transition starts.

I have never worked on nor have seen a Holley w/such large ifr's for such a mild combo.
Where are your idle mix screws set at?

Have you ever thought of dropping the ifr's to .035 or so and trying a .067 air bleed or less as a test?

[My427stang]

I was amazed too, but this came this way out of the box. Its a 1000 cfm HP series vac sec.

I dont have any official Holley specs, but I do agree, it seems very odd.

Anyone on here have access to what Holley supposedly delivers with? Maybe I got a Friday carb LOL