Need experts on Holley tuning:
I have a 750 Mech sec holley HP carb. JETs 73/79 on a 383 built with 10.3-1, Comp HYD roller cam Duration at .050 236/242, .363/.3610, gross lift .544/541, duration at .006 286/294, Lobe separation 112/installed at 108. Timing 18 initial. 36 all in.

I tuned her to max idle vacuum on 4 corner mixture and the idle mixture screws are all at about 1/4 to 3/8 from seated.
I have squared the primary transfer slot to .020 and opened the secondary idle. so that seems to be perfect.
My idle is around 13.2 AFR.
I would like to lean it out some more:( exhaust smell is there and it's a convertible)
Since the idle air bleeds are adjustable on top of the carb, can I just change those?
Or do I have to restrict the Idle restrictors in the metering block? I really don't want to put loose wires in the carb block or do a great deal of drilling if I don't have to.

My AFR #s,
IDLE 13.2
part throttle cruise:
13.7
part throttle acceleration.
14-15.5
WOT 11.0 (need to work on that)

When I last had that exhaust smell I gave it a few more degrees of initial timing. Cleaned it right up.

I think the only way to find the answer on any combination is a bit of experimenting. You have a good baseline.
With that much cam, it might take a couple more degrees initial. Try it, see if yours stop watering. If not, go back.

On the carb, you can try lowering the fuel level in the bowls a couple of flats (1/3 turn).
I'd probably start with trying to reduce the secondaries contribution at idle. A bit less secondary opening and then a bit more primary (.025" showing) IF needed after working with all four mix screws. 1/4 turn out indicates that the transfer slots are contributing a high percentage of the fuel at idle.

With an automatic trans, its too lean at idle if the rpm drops significantly when placed in D. What the AFR is in N (or P) isn't too important. Since there is no load on the engine, it can be very lean and very advanced but is useless when placed in gear.

Some questions about your AFR #s to help isolate the circuits needing adjustment:
Is this with pump gas (up to 10% eth)?
If so, don't get too hung up on the exact number, as its not exactly the same as 'pure' petroleum gas.
Is 'cruise' at 25 mph, 45 mph or 65 mph?
At 25 and even 45 mph the idle and transition circuit provides most of the fuel. By 65, the main circuits will be providing most of the fuel.

""I tuned her to max idle vacuum on 4 corner mixture and the idle mixture screws are all at about 1/4 to 3/8 from seated.
My idle is around 13.2 AFR.
I would like to lean it out some more:( exhaust smell is there and it's a convertible)
Since the idle air bleeds are adjustable on top of the carb, can I just change those?
Or do I have to restrict the Idle restrictors in the metering block?""

Since the IAB's are easier to get too, I would suggest buying a set .005" & .010" larger and then resetting your idle A/F and test again.

Note: By doing this, you will also lean out your off idle and low rpm cruise A/F too.

Fuel: 93 octane BP pump gas (probably 10 percent eth?)
cruise: 55 ish with the pedal just barely open. 3:91 gears
seems to be the same AFR at 45, 50 , 60...but I will get more data.

Fuel in bowls are just at the bottom of the threads (shake car and it drips)

I will look at the secondaries contribution to the idle again. I believe though it is barely open and similar to the primaries (square). But I will check and adjust.
I will add a few more degrees to the timing.

It's a manual transmission.

Then I will think about changing the LSIAB a little to move the AFR curve up a little.

Do you take off the carb to change out the IABs? Is there any small gaskets, or O rings in there that can fall into the intake like the squirter has?
Thanks all for the graduate work carb discussion.

On the top of the carb are eight brass bleeds. The outer are the IABs and the inner four are the HSABs.
They just screw in/out with no gaskets. I like to take a rag or a paper towel and rip a small piece off and stuff in into the top of the carb around the air bleed that I will be changing. This will help prevent the bleed from falling down onto the butterfly and becoming a pain to fish out.

Is your carb an older one with cast metering blocks or a newer one with billet blocks? The billet blocks typically are tapped and threaded for screw in idle fuel jets. If you have the screw in idle fuel jets then I'd change them. If not, change the idle air bleeds.

To change the AF ratio at WOT you'll need to change the size of the power valve restriction. Once again, look at your metering blocks to see if you have screw in PVCR jets. If you don't then I'd think about getting some billet blocks.

The easy way to tune a Holley is to get a set of billet blocks with screw in jets for each circuit. Then you can dial it right in. If you need to buy blocks I'd highly recommend giving BLP a call. The guys at BLP virtually invented the concept of Holley carb tuning and they sell all of the necessary parts.

Thanks,
I definitely don't want to drop anything down the carb. Sounds like a good idea to put something there or take the carb off completely.
Brand new Carb HP. So I will look for the idle fuel jets.

I will look up BLP thanks,

Then if the AFR meter shows a little richer than you think, its OK. That said, go by how the engine responds, not the numbers. Use the AFR as ref points to track changes, not as targets.
Throttle barely open means its mostly on the idle circuit. Because there is overlap when the idle circuit is tapering off and the mains are coming on, its best to focus check at lower speeds and higher speeds as well. Try steady cruise on flat or slight uphill at 25, 35 and something between 65 and 75 mph.

Another thing you can do to find out what is what is to lean out the main jets. Drop down two to three jet sizes. Then see what cruise speeds the AFR changes. Worst that will happen is the engine lean surges when the speed gets high enough. On my car I found that to be 60-65 mph. So I had to drive 55-60 max the rest of the way back home. This was before affordable WBO2 dataloggers so it was a very informative experience.

This is correct starting point (on old style bowls) but you can adjust this to trim the fuel contribution. I'd take out of the back first. On the bowls that come with a big sight glass, middle of the glass is correct.

There's no absolute rules here. On some carbs the slots are higher up in secondaries so are not showing at all at idle. On those you definately don't want to open the secondary throttles enough to show the slots.*

Whatever you do, make one change at a time and keep a notebook!
Working the idle/transition circuit first is always best, although not always practical for us non-pros. Get the WOT AFR on a track or dyno in at least 3rd, preferably 4th gear. Then decide if the PVCRs and secondary jetting should be adjusted.

* see Tuner's comments in this circle track carb thread for more on this topic.

There is an older carb tuning article that I wrote for MM in the tech archives. It has some pictures of the Holley billet metering blocks and some tuning info:

http://www.hotrod.com/how-to/engine/mopp-0709-cam-carburetor-modifications/

I agree,I have tried not to focus too much on the AFR exactly. I am trying to build a good picture from idle to WOT. Drive-ability etc.

As for the fuel bowls....The new HP carb I have was pouring out when I opened the sight plug. So I lowered the fuel level to just at the bottom threads. Is this correct for a new Holley? I am running 6 psi at the regulator from my electric pump. Any science to be had on how float levels affect the AFR curve? should I bring the fuel level back up in the bowls to spill out of the hole?

I assume I have the older style metering blocks where I can't change out the IFRs or the PVCR.
After reading about the adjustability, I may change to billet.

So if If I change the LS Air Bleeds on top instead of the Idle fuel jets in the block (since that is the only adjustable component I have for now), are they interchangeable? Do they do they same things except in opposite concepts?
Go bigger air bleeds and go leaner, or go smaller on Idle jets it will do the same go leaner? Or does the curve change differently for Air Bleeds as compared to Idle jet changes? Is it better to change Idle jets in my case to get the mixture screws back to manageable or will air bleed change do the same?

Great discussion, Thanks!

I know the air bleeds are handy, but I agree that the place to start with the idle mix is with the IFRs. If the initial adjustments are in the ball park, then there is so much fuel at idle that you almost have to shut it off. Which is why the screws are almost all the way in.

I will try this suggestion because it is easiest, but there are other ways to experiment, they are just more complex. Strip some stranded wire and find some in .008 range. Cut a piece like 2" or 2 1/2" long. Insert one end into one IFR and the other end into the other one. It CANNOT go anywhere and will make a difference in the idle mix for better or worse. It is just quick, effective, and easy. Or you an drill, tap, and invest in the tiny bleeds and jets just to experiment.

There is no magic A/F ratio for idle, but low 13s is usually a bit rich. If it is comfortable with 14ish at idle, it would certainly smell less.

You will need to determine if you are cruising on the idle/transition circuit or on the mains. Just tape over the booster feeds in the main body and take it for a short drive. You will know exactly when the idle/transitions shut off and which circuit you are cruising on. That will determine where to tune the cruise.

Is the cam a bit choppy at idle? The specs would indicate so. The choppier the idle, the more lead it will like. So, if the idle timing is 18*, try something like 24* and see how it likes it.

Just throwing that out there.

:thumbsup:

Your in the ballpark. I do not know if it is where they recommend starting with your particular carb. Do a search of Holley website for the list number and usually
Yes. Raising the floats raises the pressure differential (aka head pressure). Not if you want to make the AFR a little leaner. More head pressure would make a bit richer.

Or you can tapp your older blocks for set screws. see
Racingfuelsystems forum for info on help on how to do this.


As long as a little air is introduced, larger air bleed will make it leaner. However sometimes the opposite occurs in the begining of the curve. ( see graph graph from Obert for how bleeds normaly correct the AFR) Note that is for a main circuit where fuel is pulled due to the increasing velocity through the venturi. My personal experience has been that low speed air bleeds do effect idle. So you could make small changes there and see what happens.

here is the new data update: Thanks for the expert advice and inputs.
-Adjusted the timing to 36 all in. it was about 33-34.
-Idle went up so I lowered the secondary idle adjustment down a tad (mixture adjustment still best vacuum at 3/8 turn from seated).
Drove on a straight road at lower speeds and found the exact changes from idle to transition to primaries.
Idle still around 13.2 AFR
transition (transition 1/4+ throttle). 14.0 AFR
Primaries at the secondary linkage 14.0 increasing to 15ish.
WOT 12.0
Car runs smooth at all rpms. no surges or bogs.

Idle still a little rich. I need to work on that. I will work on the metering block Idle feeds. Or maybe a try a quick change of idle bleed airs. I think the idle feeds are my best bet for the curve I have.

I hate to screw up a good thing. But I think the idle is still smells slightly rich. Although, maybe a bit better with advanced timing. Hard to say.
Thanks again for the graduate level discussion.

""(mixture adjustment still best vacuum at 3/8 turn from seated).""
""Idle still a little rich. I need to work on that.""
- Now test larger IAB's (front & rear apx .005" each). This will lean out your idle and you will need to readjust your metering screws which will bring them outward (richer). Or, test smaller IFR's (front & rear .003" or so) which you will need to readjust your metering screws which will bring them outward (richer).
- Once this change is made, you will want to retest your low rpm cruise A/F (30mph-50mph). Once you are happy, then move onto your higher rpm cruising A/F (50mph-75mph etc.) Just write these A/F numbers down for reference (50mph,55,60,65,70 etc.). This will be your main jetting.
- E10 should be fine in the low to mid 14's for cruising, but watch it if the cruising approaches 15 A/F.
- After your 50+ cruising A/F is good: If you are cruising in the 14's and you push the accel pedal down slowly until your vacuum drops below your PV rating, you may want to tune your PVCR's to get you in the 13's (no secondaries. Just primary tuning).

""WOT 12.0""
- This is a little rich but the changes that you will be doing to lean out the idle/transition circuit will lean this out too. Wait to tune this until you are happy with the IFRs, IABs, Main jets, PVCR's, Primary pump cam/squirter.

Please write down every change and keep it for reference (including outside temps).

Nice job on your tuning!!

sounds like a great plan. I will order some IABs (easiest to change first). Then work the tune again. I can always go backwards.
Thanks!
I really appreciate the education. Never stop learning!

LSIAB primary is 71
LSIAB secondary is 28.

I go up on each I assume?
76/33
leaving the High speeds alone for now.

The 71's are the IABs and the 28's are the HSABs. You will have two of each in the front (Primary side) and two of each in the rear (Secondary side). You will need to buy (4) jets at a time of a certain size. For now, I would suggest buying .075's [first test], and also .078's or .080's for more testing if needed and leaving your .028 HSABs alone for now.

Note: The IAB's are the four "outer" bleeds. The HSAB's are the four "inner" bleeds (next to the squirters). Please reconfirm the location of your existing .071 and .028 bleeds. From what I have noticed, the bleeds are usually the same size from the factory front to back.

LSIABs are not squared on my 750 HP out of the box.
71/28. (Confirmed with Holley that not a production mistake)
HSIABS in secondary 36

Thanks

OK. The 028 & 036 just look like the typical HSAB sizes.
Note: Typically I see IABs in the .050"-.080" range and HSABs in the .025"-.039" range.

Then I would suggest you buy a set of .005" leaner on both for first test, then maybe another set an additional .003-.005" for possible testing. As the outside temps rise and fall, you may need a few sets in your toolbox to keep the A/F happy through out the year. You wont know what sizes you will need until you figure out what you need now, and what you will need on the coldest and the hottest normal day that you drive. I have found that I need two different sets, one set for cooler days (Spring/Fall) and one set for Summer heat.