Still working my carb tuning. My highway cruise was around 13.5, so I leaned out the jets and got it to about 14.6 - 14.9, but now I have a lean spot on light acceleration after about 2000rpm. I put the original jets back in, but the problem is still there, just much less pronounced so that you barely notice. If I lightly/mildly accelerate the afr goes somewhere around 17.5 - 19.x and the car stumbles a bunch, but if I press on the pedal some more then it goes to around 13.x and accelerates like it should.

My power valve is closing around 17 - 18 in hg, and the problem occurs from about cruise vacuum (~23) until about 17 in hg, which leads me to believe that it's a power valve opening too late. But with that strong of a power valve, do I really want to go any heavier?

Or is there some other flaw that I am overlooking? Thanks.

Bone stock 5.3
63 jets (67 originally)
IFR - .026
IAB - 55
MAB - 33
PV opens at 17 - 18 in hg
.055 PVCR
.033 squirter, blue cam in #2

Secondaries disconnected

 

Its the accelerator pump

The PV is working like it should, you just have a lean tip-in

 

Chances are an accel pump won't fix a lean issue with light acceleration.

I would first adjust the idle feed restrictor and idle air bleeds. At that RPM you are still on the intermediate circuit, the main jets really aren't contributing (much). Go a step or two larger on the IFR and maybe down a size on the idle air bleeds.

My car did the same thing until I adjusted the IFR. Was able to dump the 50 cc for a 30 cc accel pump and got more response out of the idle mixture screws once the IFR and IAB's were right.

 

the op said, when he changed the main jets, it gave him the a/f adjustment. So the main jets are contributing to his cruising a/f ratio. So why do you say then, that at that rpm, the main jets are not contributing? What % would you say that they are contributing then, if it is less than a significant amount? The op never mentioned what jet sizes he was using.

 

The main jets affect everything. BUT they are set to get WOT operation right.

Below 2-3 thousand RPM there is insufficient air flow through the venturi boosters to meter fuel. In this region fuel is metered through the transition slot via the IFR, which in turn is fed from the main well which in turn is fed by fuel from the main jets. So yes, changing the jets can affect the transition circuits but since the jets are typically so much larger than the IFR they have "minimal" effect. What most guys do is set the main jets to get WOT right, than attack the part throttle tuning.

The OP did post his carb specifics, not that it matters to what I am saying.

This article isn't half bad

Here is a more detailed one, although I seem to remember speedtigger having an issue with it. Still explains the basic concepts in more detail.

 

I always thought the main jets were for cruising, and the power valve is what dropped the a/f ratio during WOT to richen things up. Why would I jet my engine rich with the jets for wot, and THEN use a power valve to even further richen the mixture for wot again?

 

Thanks guys. I appreciate the input.

As far as the pump shot goes, the blue in #2 in the most aggressive. I just don't know if it's running out by the time I'm around ~2500 rpm. The brown cam makes the most sense, giving a steady consistent shot throughout the whole throttle movement.

I've read the second article by innovative, but I never ventured to make such extreme changes. Perhaps I should use the much larger MAB like they do in order to delay the main circuit (which I believe is definitely contributing).

 

Assuming you are LIGHTLY accelerating it's probably not the accelerator pump. It is used for more aggressive throttle movement. Try what pop n wood said about the IFR. I have had the same situation you had and this cured it

 

Ok, so if we assume that I'm still running on mostly transition, but some main jet, then it's possible that at this speed the idle circuit is used up and is dying out and the jets are just beginning to take effect. So I could decrease the IFR to a .024 from a .026, but instead of decreasing the IAB the equivalent to .050", I could take it down to something like .045". This would make it a tad richer but also keep the idle circuit coming on strong later into the rpm range. Then I guess just increase the MAB so that it starts later after where I normally cruise.

Does this sound like it will work?

 

Yes, you need to adjust both. Previous post was getting too long.

you can trade them off. Read somewhere this is why double pumpers get poor mileage. They purposely use smaller power valve control restriction and bigger jets, that they do this cause it is better for performance. The same book said opening up the PVCR allows smaller jets and better mileage in normal driving.

 

I went ahead and changed to a .020" IFR and a .035" IAB to mimic the innovative article's experiment. I also upped the MAB to .078" so that it would come on later. Didn't have any problems accelerating on the highway, although my 2800rpm cruise was a little rich at 14.0. Also, now I was having a problem with my low rpm cruise being too lean. So I changed the IAB to a .031" but it was still too lean, and I'm not sure if it got worse or not.

Should I try a .028"or .026" IAB with the .020" IFR? I feel like this is going too far and that it won't really help the situation.

For the time being (since I need to use the car) I went back to a .024" IFR and a .035" IAB. Now it was a bit too rich, so I went up to a .040" IAB and it's a little better.

I guess a .022" IFR and a .031 or .035 IAB is going to be the sweet spot for my setup. Unfourtunately it's the only bit size that I didn't order from Mcmaster...

 

If you were lean at cruise than I think you went the wrong way on the IFR. You needed to make it larger. You started with a 0.026, I would try a 0.30

As for airbleeds, a larger airbleed will make the circuit lean out as RPM (i.e. airflow) increases. So if you are a little rich coming off idle and start to lean out just before the carb switches to the main circuits, than you can try a smaller IAB.

Same thing is true for main jets/PV and HSAB. Set the jets to get the right AFR at your early RPM than adjust the bleed to keep it flat out to WOT

 

I was under a slightly different ideology, I could be wrong, but I'll explain anyway:

-Smaller air bleeds allow the fuel to start flowing earlier and keep it flowing more consistently, but allow less of a maximum flow/volume of fuel at upper rpm/throttle

-Larger air bleeds delay the flow of fuel until later (interpret as higher vacuum difference between the atmosphere and the jet), but make the mixture richer as it has more potential for fuel flow.

I'm thinking of it as pressure vs. area vs. signal strength. The small air bleed has minimal area (read as force when the atmospheric air pressure over the area of the orifice pushes fuel into the system, so a limited amount of fuel that it can push through) but has a strong signal at both high and low vacuum (pressure differential) due to it's smaller area.

The larger bleed has greater area, so at low vacuum (pressure differential) the force pushing the fuel is low, but at high vacuum, the pressure is high, and since there is more area, there is more force pushing the fuel, and greater fuel flow.

So as I see it there are two ways to get the same results:
-Decrease both the jet and the bleed (more restriction but stronger signal)
-Larger jet and larger bleed (less restriction but weaker signal)

So you have the conventional Holley way, with large IAB and small MAB
Or the Innovative way (no pun intended), with small IAB and large MAB

My process was to find the smallest IFR that would still allow a proper mixture and then decrease the IAB to keep the signal strong and flow consistent in order to keep the idle circuit active throughout the whole cruise range (800 - 3000 rpm) instead of allowing it to partially run on the mains at higher rpm cruise. Then adjust the main jet and MAB accordingly so that it comes on later after the cruise is over. It worked for the innovative tech people who did it, but who knows what else they did behind the scenes that made it possible. I'd be curious to know why speedtigger didn't prefer this method, maybe he encountered the same problems that I did.

 

Sometimes this problem is as simple as going to a lighter sprung power valve. What power valve is in the car now?

 

You lost me a bit there.

The way I think of air bleeds is they keep the mixture from getting too rich as air flow increases. The pressure drop in a venturi goes up with the square of the air speed (Bernoulli's equation), while the fuel flow through a restriction (jet) goes up linearly with the pressure drop. So doubling the airflow causes 4 times as much pressure drop hence 4 times the fuel flow through the jet.

To get around this they added air bleeds to mix air with the fuel (called an emulsion) basically reducing it's density. Get everything just right and the AFR should stay the same as air flow increases.

So my approach is to set the IFR/Jet to get the AFR right at the low end of the circuit's operating range, then see what AFR does as airflow (RPM) increases. If it starts going rich, than you need bigger bleeds. If it goes lean, smaller bleeds.

And from what I have seen anytime you change a bleed it can affect the jet/IFR, so it takes some trial and error.

I see lots of articles talking about the bleeds changing the timing of when the circuit comes in. I'm not sure I understand how to use that information. Maybe the bleeds only stay flat over a small range of airflows, I don't know. I guess if you go too big/small on a bleed than that can shift the circuit operating band up or down in RPM.

 

Right now I have a custom power valve that opens around 18" vacuum. I have another custom one that opens around 14-15" vacuum. But wouldn't I want a heavier one if It's going lean on load?

Thanks for that explanation Popnwood. I've never heard/thought of it that way before, but I'll try it out. The interesting thing about the pressure drop vs. rpm is that with the idle circuit, it's consistent since I'm pulling about 22" at idle and between 21" and 23" at 2800-3000rpm cruise.

 

Yes. Heavier spring. Sorry. If you already have a power valve the opens really early and it still goes lean, then you will have to either go bigger on the main jet or increase your IFR depending on where it goes lean. If it is lean just over idle and at very low speed cruise then it is the IFR that I would look at. If it goes lean at moderate to heavier part throttle acceleration, then I would increase the main jet and then reduce the PVCR to get the WOT AFR back.

 

So, it turns out that the problem was the main jets. No matter how large/small the IFR or IAB were, it still stumbled. I ran PVCR that were huge (.055, pretty big for this setup) with a power valve that opened early (possibly too early) and MAB all the way down to .026 and it still had a tiny stumble even with a #66 jet. I put the #67 back in and it was gone. Even with a .035 PVRC and a weaker power valve, still no stumble. Ran the MAB from .033 all the way up to a huge .078 and still no stumble.

On the flip side, now my high speed cruise (2500 - 3000) is too rich.

I started with .033 MAB and I could keep around 14.5 up until about 2500 and then it started dropping and it was about 13.5 around 3000. I was shooting for about 14.8 - 15.2 at that speed, so I put in a .040 MAB. That didn't help, and to be honest it seemed like it got richer. So I put in .055 and it was definitely richer at that speed, around low 13's at the most. I put in a weaker power valve (just to make sure that it wasn't contributing) and jumped up to the .078 MAB, and now my 2500+ rpm cruise was high 12's. Put in .029 just to see what happens and it was better, but not by much, if at all.

Low rpm (1000 - 2200) cruise is good, maybe even a tad lean. It stays around 14.8-15.5 and occasionally touches 16, but it feels ok and I'm ok with it.

Any ideas? This is kinda riving me nuts. Thanks

 

How big of an IFR did you try?

 

I went up to a .031 IFR, with a .078 IAB. It was a touch lean, so I guess I could have gone up to a .032, but the larger IAB seems to be a weaker signal and less consistent, so I tried to get it back closer to stock. Right now I have a .028 IFR and a .067 IAB.