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Carb Super tuning---help? #229
05/24/03 10:57 PM
05/24/03 10:57 PM

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Lets assume we are working with a Holley 4 barrel carb that has annular boosters, two accellerator pumps and removable air bleeds, and everything is in proper working order (ie: no blow power valves, fuel level correct, fuel delivery sufficiant, etc). The mixture is rich (12:1 or less)through all part throttle operation- idle (expected due to "big" cam), cruise(2000-2500 1/3 throttle), hi speed cruise (2500-3500 1/2-2/3 throttle). At wide open throttle it goes stoichiometric (aprox 14:1-15:1).
What would you do?

Re: Carb Super tuning---help? #230
05/24/03 11:22 PM
05/24/03 11:22 PM

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if it's a two circuit carb, close up the secondary air bleed about .008-.010 and jet accordingly after that. you will need to jet down obviously, but more so on the primary side. it's trial and error but should get you pretty close. if it's a 3 circuit carb, that's a bit different and requires some better info.

Re: Carb Super tuning---help? #231
05/24/03 11:27 PM
05/24/03 11:27 PM

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Ok, lets assume it is an unmodified 4150 HP 1000cfm carb with annual boosters- I'm fairly sure that that is a two circuit.
DRAM- whats your opnion on what the reading should be? I'm thinking that the current readings are pretty much backward from ideal, except idle, of course. All of the rich reading should be leaner, and the WOT should be richer, right?

Re: Carb Super tuning---help? #232
05/24/03 11:29 PM
05/24/03 11:29 PM
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Dram is right; the hi speed circuit(bleeds)are too large in i.d. which will give you the lean wot
fuel/air ratio. .005"-.008" smaller id's on the hi speed bleeds and possible rejetting of 2 to three sizes larger should remedy the lean wot circuit, DAVID.


ThermoQuads:stock, street/strip and race; 800/850 cfm's, 860/910 cfm's and 1,000 to 1,060 cfm's. New parts for all OEM 1971-1984, Competition Series and SuperQuad carbs and custom fabricating. Carbs bought and sold. Consulting on pre-purchases so you get the right carb the first time.
Re: Carb Super tuning---help? [Re: DEMONSIZZLER] #233
05/24/03 11:33 PM
05/24/03 11:33 PM

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Ok, one of you says jet down, one says up... I guess the bleeds would have to changed first as they can effect WOT as well as part throttle ?

Re: Carb Super tuning---help? #234
05/24/03 11:36 PM
05/24/03 11:36 PM

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basically at a steady cruise, stoich or slightly rich, 14.0 or richer is fine. for part throttle you need to be a little more on the rich side normally but it really depends on the rest of the combo and how efficient it is. your right in that the WOT A/F should be in the 12.5-1 range and the rest under part throttle light to medium load should be in the, well, let's say 13.5-14.5-1 range.

Re: Carb Super tuning---help? #235
05/24/03 11:37 PM
05/24/03 11:37 PM
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DRAM said to jet it down because of the 'rich' main circuit(not the hi speed circuit at wot)which would suggest that the primary jets(or main bleeds are too small in i.d.) are too big.


ThermoQuads:stock, street/strip and race; 800/850 cfm's, 860/910 cfm's and 1,000 to 1,060 cfm's. New parts for all OEM 1971-1984, Competition Series and SuperQuad carbs and custom fabricating. Carbs bought and sold. Consulting on pre-purchases so you get the right carb the first time.
Re: Carb Super tuning---help? #236
05/24/03 11:37 PM
05/24/03 11:37 PM
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lets assume this is your 1000hp annular, on your motor

the idle, part throttle/rich problem is mostly due to the cam being too....small.
the carb it set up for a more radical cam, which would have less vacuum, and along with that...less signal too the carb.

you need to numb up the low speed circuits because the signal is good.
i would open the idle air bleeds to about .065, and increase the PVCR by about .010 from what it is now.
if the IFR's are larger than .034 -.035...they will likely need to be choked down a bit as well.
after you increase the PVCR, you can take some jet out of it.

actually....this is the "proper" way to determine the correct PVCR sizing for your combo.....
im also assuming that since you know where the rich conditions are occuring, that you are using an in-car A/F monitor of some sort.

after doing the idle bleeds, and IFR restrictions(if necessary), jet down the primary side until the part throttle seems good.
at this point, see if there is any tip-in "issues", but not anything that would require PV enrichment.
if there are no adverse part throttle, low speed side affects from the smaller jets....then you would start to increase the size of the PVCR until the WOT mixture was back where you wanted it.

and if it were me....i wouldnt be too happy with 14.5-15:1 at WOT. IMO...thats too lean.


68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
Re: Carb Super tuning---help? [Re: DEMONSIZZLER] #237
05/24/03 11:40 PM
05/24/03 11:40 PM
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The 'bleeds' do alter the fuel curve and 'richness' but more importantly effect the 'timing' of the circuit:
the larger the i.d., the later the circuit activates and conversely, the smaller the i.d. the sooner the circuit activates.


ThermoQuads:stock, street/strip and race; 800/850 cfm's, 860/910 cfm's and 1,000 to 1,060 cfm's. New parts for all OEM 1971-1984, Competition Series and SuperQuad carbs and custom fabricating. Carbs bought and sold. Consulting on pre-purchases so you get the right carb the first time.
Re: Carb Super tuning---help? #238
05/24/03 11:41 PM
05/24/03 11:41 PM

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greg, i believe you'll still have to jet down, but like i said, more so on the primary side. i would just make the air bleed change and see where it puts you. basically your going to richen up the high speed but in order to find the proper balance between the two, you'll need to jet down a couple numbers at least. if you don't, you'll still be way rich at part throttle.
i'll just add this also, your not complaining of any other issues with the carb other than the A/F ratio. what i've posted is a 5 minute procedure. i'm of the opinion that it doesn't alway's have to be complicated. the advantage here is that you have three opinions on how to fix your carb, LOL. if the five minute procedure doesn't work, you can try the other way. i respect the other opinions posted.

Re: Carb Super tuning---help? #239
05/24/03 11:46 PM
05/24/03 11:46 PM
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i agree that some front/rear stagger jetting will be beneficial as well.
take some out of the front, and add it to the rear.



68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
Re: Carb Super tuning---help? [Re: fast68plymouth] #240
05/25/03 12:02 AM
05/25/03 12:02 AM

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"im also assuming that since you know where the rich conditions are occuring, that you are using an in-car A/F monitor of some sort."

Yep, I'm watching the people I pass to see if their eyes water or not... It's a Cyberdyne gauge, I was not sure it was working until the A/F ratio rose at WOT. Until then the whole darn gauge was lit. I decided it was time to actually start to tune this thing. Hey, maybe I'll even adjust the timing at a test and tune...

I had to look this up, even though i was just reading about it...
PVCR- power valve channel restriction
IFR-idle feed restriction

The Holley book I have does not get into the type of tuning you guys are suggesting, and I correctly thought i needed. A decription or better yet, a picture of what bleeds are what, and where the PVCR and IFR are located would be cool...


Re: Carb Super tuning---help? #241
05/25/03 12:14 AM
05/25/03 12:14 AM
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PVCR is behind the PV....the "mod" for that is...drilling.
AED makes a kit to install some inserts if you go too far

on an HP metering block, IIRC, the IFR's are in the crossover at the top of the metering block. they would be little brass insterts pressed in at an angle.
if they arent there, they are just below the emulsion trench.
making them "smaller" is done by either replacing them, or placing a small piece of wire in them as a restriction.

since the HP1000 is much more of a "race" carb than a std 850DP, the low speed circuitry is set up for a more radical motor...and for use in race conditions.
a slightly rich low speed circuit can often times help with the throttle response.

the lean WOT condition would likely also be "cured" if the carb were on a high compression 500ci motor, using much more air.
the added air flow would really get the fuel moving through the main circuit.

another simple little trick that helps lean out the low speed areas of operation is lowering the float level about 1/2 a turn.
just be sure to put it back where its supposed to be when youre racing.


68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads

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