Question for Holley jetting pros.

[TrailBeast]

Or at least someone who knows Holley's better than I do when it comes to jet changes.

Yesterday I swapped my Edelbrock 1406 out for a Holley Avenger 770 with vacuum secondaries and my air fuel ratio is a ways off.
Any guesses as to how many jet sizes I need to drop if my A/F ratio is in the 10's or high 9's?
I have the idle A/F set where I want it, but anything off idle and all through the throttle goes into the low numbers.
Right now the carb has 72 primary jets and 75 secondaries.
How many jet sizes do you think I need to drop to get it around 13-14?

Thanks all.

 

[318willrun]

Usually the Street Avengers are a tad lean vs the standard Holley 750. You might want to start with the Power Valve.... If you drop your jets, I'd only drop your primaries to 70. I think I have 72/80 jets in my 750 Holley and it burns super clean.

 

[Snake]

whats your vacuum reading in gear at idle?770 factory power valve is 6.5.

 

[Mattax]

I agree with Snake's approach^^ because what you wrote here

I have the idle A/F set where I want it, but anything off idle

Off idle is still idle circuit.

Right now the carb has 72 primary jets and 75 secondaries.

Main jets take over somewhere around 60 mph, 3000 rpm depending on car, gearing, etc etc.
Cruise at 65 mph. If your meter still indicates it 10:1 then come back and drop the primary main jets two sizes to 70. Take it back out and see how much it changes. Repeat until the engine surges. Then slow down, drive hoime and go back up 1 size.

Its worth mentioning that I've seen some reputable guys on RFS post odd metering block configurations on some of the Avenger carbs. So without seeing the block, no way to be confident things will do as they should.

All that said, off-idle should be dealt with first. Get the off-idle where the engine likes it. Idle itself can be trimmed with the mixture screws. For off-idle, work with the throttle position on transition slot first. If that takes care of it, then you won't have to mess with idle feed restriction or idle air bleeds.

 

[TrailBeast]

whats your vacuum reading in gear at idle?770 factory power valve is 6.5.

The power valve is a 6.5 and I have right at 14 manifold vacuum.
(should be good there, no?)

I agree with Snake's approach^^ because what you wrote here

Off idle is still idle circuit.

Main jets take over somewhere around 60 mph, 3000 rpm depending on car, gearing, etc etc.
Cruise at 65 mph. If your meter still indicates it 10:1 then come back and drop the primary main jets two sizes to 70. Take it back out and see how much it changes. Repeat until the engine surges. Then slow down, drive hoime and go back up 1 size.

Its worth mentioning that I've seen some reputable guys on RFS post odd metering block configurations on some of the Avenger carbs. So without seeing the block, no way to be confident things will do as they should.

All that said, off-idle should be dealt with first. Get the off-idle where the engine likes it. Idle itself can be trimmed with the mixture screws. For off-idle, work with the throttle position on transition slot first. If that takes care of it, then you won't have to mess with idle feed restriction or idle air bleeds.

Did the drive yesterday up to around 80mph, as well as some WOT tests and 35-45 cruising and the A/F ratio was always in the high 9's to low 10's throughout the entire throttle range.

I didn't mean exactly off idle.
What I meant was anywhere after the throttle stop including all other throttle ranges.
Guess I should have said everywhere but at idle.
I was thinking 2 jet sizes as well, but not up on the Holley like I am on the Edelbrocks, and I didn't really want to end up buying 12 different jets to get it into the high 13's to low 14's.

The thing runs great throughout and I have no complaints, but just a bit rich.

 

[Snake]

Hollys run rich........ try drilling the primary throttle blades that will sure clean things up,i would jet down to 70. i always jet my fronts to 69 a tad lean,just dont want to push too much fuel to the oil.ps your vacuum is perfect.

 

[Mattax]

The power valve is a 6.5 and I have right at 14 manifold vacuum.
(should be good there, no?)

If that's manifold vacuum at idle, its not really related to PV selection.
The PV number is like the step up spring in the Carter.
If the the engine has good part throttle fuel distribution and efficiency, a low opening like 6.5 might be OK. Looking at some of the Holley 4bbls used by Chrysler in the late 60s, most used 8.5 PVs. The Holley 3310 used by GM on some of their hottest factory engines used a 10.5 PV on the primary side.

Do you have a way to measure vacuum or MAP when driving the car?
As the throttle is opened slowly from a steady cruise, you'll see the vacuum drop.
AFR should go leaner and the engine should pull harder (accelerate more if on a flat road).
At some throttle opening and manifold vacuum, the fuel mixture needs to go richer for the engine to pull harder.
If you feel a flat spot, or a flat and then it goes as you open it further, definately try a higher PV opening point.

Did the drive yesterday up to around 80mph, as well as some WOT tests and 35-45 cruising and the A/F ratio was always in the high 9's to low 10's throughout the entire throttle range.

The fact the AFR stayed the similar under all conditions makes me suspect the PV may not be sealed against the block, or something like that.

I didn't mean exactly off idle.
What I meant was anywhere after the throttle stop including all other throttle ranges.
Guess I should have said everywhere but at idle.

Oh you described it perfectly clearly.
What is not clear is the terminology used in the carburetor world.
The idle system shouldn't be called that. The 'idle system' supplies the fuel under all low throttle conditions.
The main system only comes into action when there is enough air velocity in the venturi. At low throttle, whether its idle, or cruising at 40 mph, doesn't pull much air. Not enough to create a strong pressure drop at the booster wall. But since there is a strong vacuum under the throttle, that vacuum acts on the transition slot - which pulls fuel from the idle downwell.

 

[j par]

.. interesting...

 

[crackedback]

Look for fuel dripping from the boosters.

Make sure nothing is plugging up the air bleeds.

 

[Daves69]

....suspect the PV may not be sealed against the block, or something like that.

Wrong gasket or offset and not sealing possibly?
I just went through a used 3310 that I bought a while back. It ran "poorly" what was told. I found the PV was likely cross threaded when installed into the block.

...I didn't really want to end up buying 12 different jets....

On the bright side, if you buy the 70's for the primary, ya' won't need to by 72's for the secondary.

 

[yellow rose]

Wrong gasket or offset and not sealing possibly?
I just went through a used 3310 that I bought a while back. It ran "poorly" what was told. I found the PV was likely cross threaded when installed into the block.
View attachment 1715491727

On the bright side, if you buy the 70's for the primary, ya' won't need to by 72's for the secondary.

Just when you think you've seen it all, you come along and drop a cross threaded power valve into the mix.

I just don't know how that happens, other than just straight carelessness.

Dang

 

[TrailBeast]

If that's manifold vacuum at idle, its not really related to PV selection.
The PV number is like the step up spring in the Carter.
If the the engine has good part throttle fuel distribution and efficiency, a low opening like 6.5 might be OK. Looking at some of the Holley 4bbls used by Chrysler in the late 60s, most used 8.5 PVs. The Holley 3310 used by GM on some of their hottest factory engines used a 10.5 PV on the primary side.

Do you have a way to measure vacuum or MAP when driving the car?
As the throttle is opened slowly from a steady cruise, you'll see the vacuum drop.
AFR should go leaner and the engine should pull harder (accelerate more if on a flat road).
At some throttle opening and manifold vacuum, the fuel mixture needs to go richer for the engine to pull harder.
If you feel a flat spot, or a flat and then it goes as you open it further, definately try a higher PV opening point.




The fact the AFR stayed the similar under all conditions makes me suspect the PV may not be sealed against the block, or something like that.

Oh you described it perfectly clearly.
What is not clear is the terminology used in the carburetor world.
The idle system shouldn't be called that. The 'idle system' supplies the fuel under all low throttle conditions.
The main system only comes into action when there is enough air velocity in the venturi. At low throttle, whether its idle, or cruising at 40 mph, doesn't pull much air. Not enough to create a strong pressure drop at the booster wall. But since there is a strong vacuum under the throttle, that vacuum acts on the transition slot - which pulls fuel from the idle downwell.

I can hook up my vacuum gauge and hook it to my wiper to check it that way.
No flat spots or hesitation anywhere in the throttle range.
Thanks for the lesson.

Look for fuel dripping from the boosters.

Make sure nothing is plugging up the air bleeds.

Checked those and they are clear. (learned that one on a local 67 GTO)
The carb is super clean and no fuel dripping from the boosters like a blown power valve causes.
Thanks though.

Wrong gasket or offset and not sealing possibly?
I just went through a used 3310 that I bought a while back. It ran "poorly" what was told. I found the PV was likely cross threaded when installed into the block.
View attachment 1715491727



On the bright side, if you buy the 70's for the primary, ya' won't need to by 72's for the secondary.

True enough on not needing to buy 72's.
I have seen once in my life a cross threaded power valve and couldn't imagine how the hell?

Power valve looks brand new and I even pulled it to check the gasket behind it, so no cross threading or leaking past it.
No indication of it being blown either.
It is possible it might have a wrong gasket, but could that show up as being two or three AFR numbers rich all through the range except idle?
It starts runs pretty much perfect other than being a little rich.

I'll pick up two 68's and put those in the primaries, then put the 72's in the secondaries and see what it does.
The car this carb came off of was a running big block Roadrunner at 2,500 feet lower elevation, so it would make sense that it would be about this much richer up here where I am.
It was pulled for an EFI swap.

Thanks all.

 

[MOPAROFFICIAL]

What pump nozzle is on it, and pump cam?

72 primary isnt big, but a jet less could be the sweet spot. Try a 70 or 71 primary jet.
Make sure the pump nozzle isnt too big.
.....



But before any of that, put a stiffer spring in the vacuum secondary... it's probably opening way early.... and btw a good way to tune a vac sec carb is to toss the stiffest spring in the secondary and then tune the primary up to 3000 rpm 1st...if its clean, responsive under normal light pedal driving... move to the secondary..
.starting with moving from the stiffest spring through the lighter ones till it transitions properly/smooth to all 4.
You can short cut 2 spring tensions at a time usually

 

[Mattax]

The carb is super clean and no fuel dripping from the boosters like a blown power valve causes.

If the power valve is ruptured or not sealing, fuel goes straight down into the manifold. Unless you have a 4- hole spacer underneath, then it spreads out a bit as it goes into the plenum.
Dripping booster would be fuel level too high.

It is possible it might have a wrong gasket, but could that show up as being two or three AFR numbers rich all through the range except idle?

There are two gaskets shapes. One is slightly triangular on the inside diameter. Sometimes those gaskets split upon removal; one half sticks to the block and the other half to the power valve.

It starts runs pretty much perfect other than being a little rich.

Yea. Sure. It's rich! That's what engines like when starting.

 

[Mattax]

I agree with Mopar Official. Don't mess with the secondaries at this point. Get them out of the action for now. Not that they should be involved in anything you've tested except maybe the WOT if the rpms were over 3000.