Holley running rich at part throttle cruise

[evil66]

Just been doing some experimenting with the 750 HP Holley double on my 408 small block 69 Barracuda after reading the power valve thread. Fitted an afr gauge to check the cruise afr's.

Original setup after dyno was 74/82 jets, 6.5 power valve. Seemed to cruise rich and had a hole in the middle under acceleration but full throttle was fine.

Fitted 72 primary jets and a 10.5 inch power valve. Idle was 13.5:1, light throttle cruise was around 11.8:1, 3000 to 3500 at 60 to 70 mph was 12.4:1.

Fitted 70 primary jets. Light throttle cruise was still 11.8:1, 3000 to 3500 cruise was better at 13.0:1

Im looking at what is causing the rich condition at light hrottle cruise. Car has a 248 at 50 solid magnum cam.

Is it being caused by the idle feed restrictions and do i need to change these to.lean it out a but or is the gauge just reading unburnt fuel.

Once i get this issue sorted i will be drilling the pvcr's to get back to the right full throttle afr and may try one or two sizes smaller on the main jet to get around 13.5:1 to 14:1 cruise afr.

Any suggestions appreciated.

 

[Oldmanmopar]

Change the primary air bleeds go 2 sizes bigger or drill them if yours are not threaded.. Or you can pull yours and thread the holes , there is a kit for that. Also Drill 3/32 holes in all 4 of the venturi valves. This will make a big difference if you don't have a 4 corner idle circuit carburetor. You probably are out of square on your transfer slots.

 

[Demonic]

Could your power valve be bad?

 

[Mattax]

Original setup after dyno was 74/82 jets, 6.5 power valve. Seemed to cruise rich and had a hole in the middle under acceleration but full throttle was fine.

Do you have the AFR readouts for the pulls?

Is the hole when acclerating from a stop sign or low speed part throttle?
Or is the hole when you hit a hill or go to passing lane on the interstate?

Fitted 72 primary jets and a 10.5 inch power valve. Idle was 13.5:1, light throttle cruise was around 11.8:1, 3000 to 3500 at 60 to 70 mph was 12.4:1.

Fitted 70 primary jets. Light throttle cruise was still 11.8:1, 3000 to 3500 cruise was better at 13.0:1

That demonstrates that light throttle cruise is mostly on the 'idle circuit'. A name that is certainly is misleading.
It also shows that your combination will tolerate the leaner high speed cruise provided by the 70 PMJ (primary main jets).
What flavor 750 is this Holley? An old school 4779 or something else?

Im looking at what is causing the rich condition at light hrottle cruise.

Most likely something in the idle circuit. In particular the combination of idle restriction, idle air bleed and transfer slot exposure. That stuff OMM posted above. I don't agree on opening the idle air bleeds just yet, but that very well may end up being what you do.

Do check the transfer slot openings. I disagree with the set the primaries square as a goal. Rather set the primaries so they are roughly square, but an old school Holley can work from .020 to .040 showing. Others with different slot witdths and lengths will differ. First see where you are, then experiment with slight changes within that range.

Is it being caused by the idle feed restrictions and do i need to change these to.lean it out a but or is the gauge just reading unburnt fuel.

It's the combination of IFR and IAB and transfer slot restriction (which is a variable airbleed and fuel-air restriction depending on the throttle blade position).
I think you can assume your gage is ballpark correct until shown otherwise.

Once i get this issue sorted i will be drilling the pvcr's to get back to the right full throttle afr and may try one or two sizes smaller on the main jet to get around 13.5:1 to 14:1 cruise afr.

Aim for performance. Cruise as lean as it will tolerate. Jet for best mph or area under the curve after determining the AFR is flat through up to the shift point.

 

[Treblig]

Power valve comes into play only when you accelerate!!

 

[yellow rose]

Do you have the AFR readouts for the pulls?

Is the hole when acclerating from a stop sign or low speed part throttle?
Or is the hole when you hit a hill or go to passing lane on the interstate?





That demonstrates that light throttle cruise is mostly on the 'idle circuit'. A name that is certainly is misleading.
It also shows that your combination will tolerate the leaner high speed cruise provided by the 70 PMJ (primary main jets).
What flavor 750 is this Holley? An old school 4779 or something else?

Most likely something in the idle circuit. In particular the combination of idle restriction, idle air bleed and transfer slot exposure. That stuff OMM posted above. I don't agree on opening the idle air bleeds just yet, but that very well may end up being what you do.

Do check the transfer slot openings. I disagree with the set the primaries square as a goal. Rather set the primaries so they are roughly square, but an old school Holley can work from .020 to .040 showing. Others with different slot witdths and lengths will differ. First see where you are, then experiment with slight changes within that range.

It's the combination of IFR and IAB and transfer slot restriction (which is a variable airbleed and fuel-air restriction depending on the throttle blade position).
I think you can assume your gage is ballpark correct until shown otherwise.

Aim for performance. Cruise as lean as it will tolerate. Jet for best mph or area under the curve after determining the AFR is flat through up to the shift point.

I agree with this and I aplogize for not responding to your PM.

If you use google image and look up T slot restriction you can see which holes feed the T slot. You need to measure both passages...the short passage that is where the metering block bolts up and the vertical passage through the main body that actually goes down to the base plate and T slots themselves.

They are usually .109ish and the vertical passage may be a bit smaller. You can use an 8/32 brass set screw and use about an .080-.082 hole to start as a restriction and tune from there.

Sometimes, you can open up the top emulsion hole in the metering block to clean up the cruise a bit. A couple of thou is a start on those, but that should be your last tuning change.

 

[evil66]

Could your power valve be bad?

Power valve is new and idle mixture screws are sensitive to minor adjustments so i think its fine

 

[evil66]

Change the primary air bleeds go 2 sizes bigger or drill them if yours are not threaded.. Or you can pull yours and thread the holes , there is a kit for that. Also Drill 3/32 holes in all 4 of the venturi valves. This will make a big difference if you don't have a 4 corner idle circuit carburetor. You probably are out of square on your transfer slots.

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Carb is 750hp with four corner idle and adj air bleeds.

I will double check the transition slots they were set years ago but may have changed fiddling with idle speed.

 

[evil66]

Do you have the AFR readouts for the pulls?

Is the hole when acclerating from a stop sign or low speed part throttle?
Or is the hole when you hit a hill or go to passing lane on the interstate?





That demonstrates that light throttle cruise is mostly on the 'idle circuit'. A name that is certainly is misleading.
It also shows that your combination will tolerate the leaner high speed cruise provided by the 70 PMJ (primary main jets).
What flavor 750 is this Holley? An old school 4779 or something else?

Most likely something in the idle circuit. In particular the combination of idle restriction, idle air bleed and transfer slot exposure. That stuff OMM posted above. I don't agree on opening the idle air bleeds just yet, but that very well may end up being what you do.

Do check the transfer slot openings. I disagree with the set the primaries square as a goal. Rather set the primaries so they are roughly square, but an old school Holley can work from .020 to .040 showing. Others with different slot witdths and lengths will differ. First see where you are, then experiment with slight changes within that range.

It's the combination of IFR and IAB and transfer slot restriction (which is a variable airbleed and fuel-air restriction depending on the throttle blade position).
I think you can assume your gage is ballpark correct until shown otherwise.

Aim for performance. Cruise as lean as it will tolerate. Jet for best mph or area under the curve after determining the AFR is flat through up to the shift point.

Carb is later than a 4779, its a 750hp with adj air bleeds. I had a 4779 on it originally.

The hole originally was at low to mid speed crusing and give it a quick stab on throttle, seems much better with the 10.5" pv over the 6.5".

Wide open throttle was always good with the 74/82 jets and 6.5" pv however it was running a bit rich cruising around, so i would like to end up with the same total fuel at wot with better driveability and leaner cruise.

Havent done much wot testing with the leaner primary jets, i want to get the pvcr channels drillled and tapped so i can increase the total fuel back to total of 74 jets plus pv with the leaner main jets and bigger pvcr channels.

 

[evil66]

I agree with this and I aplogize for not responding to your PM.

If you use google image and look up T slot restriction you can see which holes feed the T slot. You need to measure both passages...the short passage that is where the metering block bolts up and the vertical passage through the main body that actually goes down to the base plate and T slots themselves.

They are usually .109ish and the vertical passage may be a bit smaller. You can use an 8/32 brass set screw and use about an .080-.082 hole to start as a restriction and tune from there.

Sometimes, you can open up the top emulsion hole in the metering block to clean up the cruise a bit. A couple of thou is a start on those, but that should be your last tuning change.

No problem yellow rose, i havent had much chance to get out to do any more tuning since the weekend.

Havent read much about limiting the t slot circuit but that would make sense as i think that is where it is running the richest before it comes on the primary jets. Is this a better method than reducing the size of the idle feed restrictors?

 

[Mattax]

Carb is later than a 4779, its a 750hp with adj air bleeds. I had a 4779 on it originally.

The hole originally was at low to mid speed crusing and give it a quick stab on throttle, seems much better with the 10.5" pv over the 6.5".

Wide open throttle was always good with the 74/82 jets and 6.5" pv however it was running a bit rich cruising around, so i would like to end up with the same total fuel at wot with better driveability and leaner cruise.

Havent done much wot testing with the leaner primary jets, i want to get the pvcr channels drillled and tapped so i can increase the total fuel back to total of 74 jets plus pv with the leaner main jets and bigger pvcr channels.

The newer carbs can be goofy. Maybe you've discussed this with YR in PMs. Extra and too much e-bleeds, kill bleeds in the wrong location, long t-slots are some of the main offenders. That all said just as a caution. In general to get the idle rich enough to be decent at relatively low vacuum and not have off-idle and low speed cruise rich takes some work.
Lets say its 8"Hg of vacuum at 800 rpm and 14"Hg at 1200 rpm in gear. That's a big change in pressure on the idle - so lots more fuel will flow even if the amount of transfer slot hadn't changed with throttle opening. For example when coasting.

Having the hole in low to mid speed is tricky to evaluate because the main system is starting to contribute fuel during part throttle acceleration. If its not happening at all at highway part throttle then its the transition off of transfer/idle or the quality of initiation of the main system.
Idle system normally will continue supplying at higher rpm/deeper throttle by using a little larger IFR. To keep the ratio similar the IAB will have to increased at the same time.
Initiation of the main system can be effected by those items mentioned sometimes 'goofy' on new carbs. Also by main airbleed being too large or too small. But the main job of the main airbleed is to correct AFR so it remains consistant through high rpm. So I like to know how that is working before change MAB. Guys with more experience can use that knowledge to jump ahead based on their knowledge of how many combinations have worked out in the past. Sometimes they can go to something similar in their notebooks.

 

[Oldmanmopar]

What I can tell you is My 416 has a 3.5 power valve 686 lift roller. My 406 has a 4.5 560 hydraulic . The last Hydraulic cam I will ever use. The removable air bleeds really work when it come to taking fuel out of my 416. They are easy to change on the car and really make a difference in changing EGT's. We bought a bunch when we were playing with this car below and they were instrumental in tuning this car. These 2.2 motors have a pull over problem. We would have spent weeks finding the correct fuel metering. Use the air bleeds that is why they made them removable

 

[evil66]

Just an update, comverted the metering blocks on the old 750hp to adjustable power valve channel restrictors and adjustable idle feed restrictors.

Dropped the ifr size from 35 thou to 31 thou and it now cruises on the transition slots at 12.8 to 12.9.

Increased the pvcr size from 61 thou to 67 thou and my wot afr is around 12.9 to 13.0 so i might go up one or two thou more.

Really happy with the leaner main jet and leaner transition circuit, cruises so much better and more responsive,will keep improving it.

 

[Mattax]

That's great.

If you saw the tables I posted yesterday, the big difference in the transition area was pIFR of .0295 to .028".
In the second one, you can also see where I point out the primary jets effect on high speed cruising.

Other differences in the main circuit between two charts; plugged the e-holes so it was like an older Holley block, made the pMAB a little smaller, and reduced main jet. Main air bleeds and e-holes can effect how smoothly and when the main circuit starts up.

 

[TrailBeast]

Just an update, comverted the metering blocks on the old 750hp to adjustable power valve channel restrictors and adjustable idle feed restrictors.

Dropped the ifr size from 35 thou to 31 thou and it now cruises on the transition slots at 12.8 to 12.9.

Increased the pvcr size from 61 thou to 67 thou and my wot afr is around 12.9 to 13.0 so i might go up one or two thou more.

Really happy with the leaner main jet and leaner transition circuit, cruises so much better and more responsive,will keep improving it.

Just an FYI, AFR gauges and 02 sensors read how much oxygen is in the exhaust gasses and not how much fuel is in it.
Not that it really matters as far as adjusting the AFR.

 

[crackedback]

AS Mattax has mentioned, try stepping the IFR down to .028 and see how things go.

 

[Mattax]

Original setup after dyno was 74/82 jets, 6.5 power valve. Seemed to cruise rich and had a hole in the middle under acceleration but full throttle was fine.

Do you have printouts or files from the dyno runs?

Increased the pvcr size from 61 thou to 67 thou and my wot afr is around 12.9 to 13.0 so i might go up one or two thou more.

I understand you prefer to err on the richer side. Might be good to hit the dyno again. Even more so if the emulsion holes and kill bleed in the 4779 blocks are different than what was tested before.
That's one of the points of this comparison
100 cfm more with anular and downleg boosters can't beat AFR
A series of incremental jet changes showed where max power was (is) for that combo.
Another is that its useful to observe the high gear WOT AFR over the full range of rpm.

 

[evil66]

AS Mattax has mentioned, try stepping the IFR down to .028 and see how things go.

Thats around where i was thinking, will give it a try.