[aj06bolt12r]

Thanks Eric, that makes sense.

I suspect that you are correct about the idea that I am cruising on the transition circuit and not the mains. Because of the fact that I am using manifold vacuum to increase timing at idle I do not have the throttle blades open very far at idle in the first place and....

The amount of throttle needed to maintain cruising speeds is very small. I feel like I'm barely touching the pedal.

I'll take your advice and see what readings I get and see how the truck feels.

 

[dwighty390]

Isn't there a PV in the secondary block on a double pumper?

 

[eric32]

No double pumper made in the last 20 or so years has a secondary power valve unless you get one that has the provision in the rear of the main body. Only the very first generation of holley double pumpers came with secondary power valves. That is on the 600 to 750 ones I am mentioning here the 4776 through 4779 models. I don't know about some of the bigger cfm ones and the dominator carbs.

 

[ericnova72]

No double pumper made in the last 20 or so years has a secondary power valve unless you get one that has the provision in the rear of the main body. Only the very first generation of holley double pumpers came with secondary power valves. That is on the 600 to 750 ones I am mentioning here the 4776 through 4779 models. I don't know about some of the bigger cfm ones and the dominator carbs.

800, 850, 950 and 1000 cfm DP's are the same, no rear power valve.

holley discontinued power valves in the rear due to the fact hard acceleration will uncover the jets and the power valve inlet....jets have a solution, the jet extension...but there is not a way to extend the power valve inlet to the rear of the bowl like a jet extension does for the jet.
I've see a plastic sleeve/cap type deal for the power valve, but no idea how good it works and since only one cmpany seems to offer it, must be not much respose for it.

Some of the superspeedway/circle track calibrated carbs still have power valve in the rear, but it is only a couple of models like the restrictor plate carbs and such.


For the most part, the primary side is the only one that has a real need for a power valve to trim the fuel curve between 1/4 throttle and WOT....the secondary is either shut or WOT, rarely are you ever in between those two points on the secondary throttle plates.

 

[TommyK]

The only application I can think of that could potentially benefit from a rear power valve is endurance road racing where you might spent significant periods of time between half and WOT.

 

[johnsongrass1]

Every Dirt track carb I can thing of has one. They also use 1:1 throttle shafts links in almost all cases aside from the guys who like vacuum secondaries on slick tracks.

 

[ericnova72]

Every Dirt track carb I can thing of has one. They also use 1:1 throttle shafts links in almost all cases aside from the guys who like vacuum secondaries on slick tracks.

Thanks for that info, I wasn't sure if more of the Circle Track oriented carbs had PV's in the rear....I find it interesting they have mostly all gone to 1:1 linkage though. Never would have guessed that would be any advantage.

 

[bentwings]

A couple things you need to do:
Get Tuning Holley Carbs Book. It will educate you on how each circuit works in the Holley carb. Each circuit works with all the other circuits and has a function.

Get a log book to note each change and results. Be analytical not wishful thinking. Write down what really happens. Don’t be afraid to retest things....some times you can even lie to yourself.

Make sure that you thouoroly warm the motor before doing any tune. It can take a solid half hour to get everything really warmed up. Please don’t tell me your temp gage reads 180 in 5 minutes. It’s not completely warmed up...you will get false readings. You can even see this on your AFR. Write readings down or just throw it away....you spent a bunch on the correct gage use it.

Add the vacuum gage no matter how hard it is. ..no excuses. You need this info too. Record it accurately

Make sure the throttle blades are in the transition slot correctly. Your cam may require more throttle opening thus uncovering the transition slot too far. You may have to open the secondary blades a little to get the slot correct.

Make big enough changes so you can see the result and feel it driving. Then you split the change for finer tuning.

 

[aj06bolt12r]

I went ahead and changed out the primary main jets today. They were 74s now they are 70s.

This was for sure a step in the right direction. Steady state cruise anywhere between 60 and 80 mph is showing high 13's to low 14's.
This is true if I am running in 4th gear or in overdrive, the AFRs stay the same. But any time I have to pull even a small grade or try to accelerate gently the AFR's drop into the 12's.

I also learned for sure that at highway speeds I am defiantly on the main jets more than the transfer slot.... After swapping out the jets I reset the idle mixture screws as well. I must have got in too much of a hurry doing this because I screwed it up... The mixture was way too rich at low speeds and idle. In the low 12's. But it cleaned up to high 13's low 14's at highway speeds.

So I whipped into a gas station and took my time setting the idle mixture screws again. AFRs were back up to high 13's now at idle and at city traffic speeds.

Pretty much I'm mostly on the main jets at speeds above 50-55mph or so and transfer slot/ idle mix screws below that.

At WOT I am now seeing high 11's, so not enough of a change there.

I am thinking I should swap out my 6.5 power valve for a 5.5 to prevent the mixture going rich for every little hill or minor increase in speed.

I also think I need to go ahead and pull 4 jet sizes out of the secondary mains. From 80's to 76's

I am not sure what to do about the rich mixture at low speeds. My idle mixture screws are only about 1/2 turn out from being all the way in as it is. Air bleed screws?

I will get the vacuum gauge on this thing tomorrow and get some readings mapped out for different situations. But I think I have a pretty good handle on what the carb is doing as is. I'm just hesitant to go messing with anything other than main jets and power valves. I have no experience past that.

 

[ericnova72]

The change from 6.5" to a 5.5" power valve will most likely be too narrow a change, I'd drop to a 4.5".

You'll likely have to reduce the size of the idle feed restrictions(IFR's) in the metering block rather than increasing the idle air bleeds(IAB's). Increasing IAB size will lean it like you think, but it also alter system timing between idle to transition to mains. It also takes bigger changes to air bleed size(4-6 sizes per adjustment) to see any results, which can further aggravate things

Decreasing the IFR's will lean the idle circuit and transition without an impact on system trade-off timing, and allow you more range of adjustment back to the idle mixture screws. Go .002-.003" smaller at a time and retest.

As is typical, I see Holley needs to be slapped.....they list all the carb specs but leave out IFR size and PVCR's diameter. Bunch of morons.

I agree with now going down 4 sizes in the secondary jetting, see what that does for WOT .

 

[aj06bolt12r]

So I made a new discovery today that has me even more confused. I made no farther changes to the carb at all. Still all stock other than the 70 main primary jets.

I replaced the spark plugs today, #7 was fouled bad. The rest of the plugs looked pretty good, a little rich. I also replaced the header and collector gasket on the passenger side. No other changes were made.

AFRs are still the same as they were last night at idle, cruise and wot.

But I made some 1/2 throttle pulls through the gears this afternoon. Just shift and go straight to 1/2 throttle from like 2k to 4500 through the gears. Im not sure I have ever done that with this new engine before. Usually just take it easy or WOT. Anyway....

What is weird is that when I do this it goes very lean, like 16's, enough to make power fall off. If I crack it wide open it goes back rich so its not a float bowl issue.

Whats going on with that? Why would it be very lean at 1/2 throttle and rich everywhere else?

 

[aj06bolt12r]

Never mind, fuel pressure regulator decided to **** itself... back to the parts store with me

 

[aj06bolt12r]

Alright new bypass fuel pressure regulator installed and set at 6.5psi. Float levels have fuel 1/2 way up sight glass.

I also replaced the rear main jets. Stock was 80 they are now 76.

WOT afrs are getting better still rich at lower rpm. If I floor it in 4th gear at about 3000 rpm so it sees a high load and cant accelerate too quickly im still seeing WOT AFRs in the mid 11's

WOT in 2nd gear and allowed to rev out results in mid 12's at around 5k to 6k rpm. Pulls hard and sounds clean.

Im tempted to lean out my WOT AFR a little more but I think Ill try to get idle and transition circuits dialed a little better before I mess with the main jets any more.

Cruising at 70mph at around 2300 rpm im looking at AFRs in the high 13's to low 14's. Not too bad. So Im thinking Im close on my primary main jets.

But very light load cruising, like inside city limits at 40-50 mph im seeing AFRs mostly in the high 12's. I believe I am still running on the idle/ transfer circuit at these speeds. I'm only 1/2 turn out on my idle mixture screws right now. So if I'm learning anything from you guys that tells me that I need to reduce the size of my idle feed restrictors.

I called Holley Tech Support and they tell me that my stock size IFR's are 33s front and back. How much of a change do you think I should make? Should I try some 30s? Should I replace all 4 at once and keep it square?

Thanks!

 

[eric32]

Replace the idle feed restrictors with .031 as that will make a big enough change to lean things out. You hav to go up or down .02 sizes at a time to get a change and only being 1/2 way out and no more on the idle mixture screws always in most instances means its too rich.

For me in my experience when you hit a more correct idle calibration on the idle/transfer circuit on a 4 corner idle carb the idle mixture screws will be around 3/4 or so on all four and yes you need to keep the front square with the rear. From mild cams up to big cams usually the idle feed restrictors usually end up being .028 on the real lean side and .031 being kind of between slightly lean but more rich and .033 and up is on the richer side of things for bigger cams.

Since you have changeable restrictors its worth a shot and try. I would not mes with air bleeds as they are not as sensitive as idle feed restrictors and are more changed for a really final touch overall on a tune. That is in my experience.

 

[dwighty390]

You might find that your main jets are a little undersized once you lean the idle circuit back out. They might also be just fine as is.

The mid 11's probably isn't a terrible thing when you are flooring it at 3k, but you might clean that up a bit by leaning out the PVR a tad.

 

[aj06bolt12r]

Yep we shall see. One change at a time a re-test. Nice to see the changes taking place.

Just for discussions sake. Have you guys read this article? Look at the DRASTIC changes they made to the size of the idle air bleeds and idle fuel restrictors... wow

https://www.innovatemotorsports.com/resources/carb_EFI_mileage.php

 

[dwighty390]

Yep we shall see. One change at a time a re-test. Nice to see the changes taking place.

Just for discussions sake. Have you guys read this article? Look at the DRASTIC changes they made to the size of the idle air bleeds and idle fuel restrictors... wow

https://www.innovatemotorsports.com/resources/carb_EFI_mileage.php

Drastic indeed. This shows you how far off things can be. I find it interesting that their initial goal is to get the "1.5 turns out" mark...I don't feel as though there is any importance to where this lands with everything else is tuned, so long as it is open. It really doesn't affect anything other than idle since the fuel can flow through the transition circuit as needed after idle. Maybe I will learn otherwise as my carb tuning continues.

It is important to recognize the difference between IFR and IAB tuning, IMO. They are not a stand-in adjustment for eachother nor are they completely unique.

 

[ericnova72]

Good article, thanks for posting it :thumbup:....I've not seen a write-up with such drastic changes but the explanation of the changes/effects/results was very informative. .

 

[TommyK]

There had to be something drastically wrong with the carb in the article to require those ifr/iab sizes.

 

[eric32]

I understand using a wide band to tune but wow I am just floored on what they did and out of all my years of tuning holley style carbs I have never had to change anything that much and talk about going way out of whack. By going down so far on the idle feed restrictors no wonder they had to make the idle air bleeds super rich just to have an idle again.

Don't get me started on the high speed air bleeds. My goodness. If a person who was new to tuning holley carbs went by that then they would be so far out of tune I would be scared for the engine.