This textbook picture (from the RFS emulsion tuning sticky) makes interesting reading. So I may want fewer holes to reduce the low-flow mains... assuming it's the lower of the two active bleeds. Could plug that one and see how it changes!

I have been concentrating on the t-slot thinking the problem was solely a slot that's too big, which is certainly part of it.
But even with a .045 TSR it still gets too rich sometimes (and too lean at others).
Now I'm thinking part of my problem is too-big main jets that start to come in (while still on the t-slot too) but lean out at higher rpm and larger throttle opening...
Which is just what I'm seeing (12.5 at 3000 rpm, 15", and 14.0 at 3000+ rpm, 10").

So going bigger on the MAB was likely the wrong thing to do!
Dropping the main jets several sizes and decreasing the MAB would reduce that contribution at low throttle openings, and it would get richer with more flow. (Then increase the PVCR as needed to maintain AFR at wide-open primary.

If THAT doesn't work, then I can start on the emulsion bleeds... thoughts?

If you have 15 inches of vacuum at cruise you NEED a 10.5 power valve. Opening the PV as late as you are puts a hole in the fuel curve just like you are getting and you have to cover it somehow.

In other words, because the PV is opening so late, you can’t cut the T slot back far enough to clean it up. Open the PV sooner and let it do it’s job and you should be able to take some fuel away from the T slots and lean it up.

Unless I’m missing something that’s what I see.

That was my plan once I got the t-slot + mains cleaned up (an earlier-opening PV and probably larger PVCR).
As I just posted, I think it's time to make the main jet smaller, and I messed up on the MAB and should've made it smaller too so it won't lean out as much with larger throttle openings.

But you're right that I'll absolutely need an earlier-opening PV especially if I reduce the main jet, since that would make the curve hole worse. I have a 9.5 handy so I'll try that first.

"Light acceleration" is a relative term with this setup in street use. Even at 10" vacuum, still on the primaries (less than 40% throttle with the progressive linkage), and in 4th gear, it accelerates quite promptly when starting at 3000 where the cam is just coming into the start of the powerband.

I hate to change multiple things at once, but I'll probably wear out the main body threads before I get all this sorted one change at a time!

OK, so: drop PMJ from 77 to 74(?); change PV to 9.5; then reduce MAB to .030 or even .028. Not sure about the .059 PVCR yet but I know they're bigger on my old 800.
How's that sound?

I would like to see you use a #75 jet size instead of going to the # 74

I’m not sure I would drop the MAB any and if you do make that change, I would do it after you make the other changes. The good news is the bowls don’t have to come off to do that!!!! If you reduce the size of the MAB you delay the booster coming on, and you will make the A/F ratio richer at higher RPM. So the bigger MAB has the opposite affect, which is a bigger MAB will get you on the booster sooner and lean the higher RPM.

The rest looks like what I would do.

agreed, i have one of these 1.45" 950 carbs and i went from 78 to 72 (expecting it to be lean) and ended up getting back up to 74 or 75 to get the cruise happy. stepped downleg boosters.
this is on a mile 360 with the normal stuff, headers, airgap intake, good compression and semi lumpy cam...

i would get the cruise jetting in the range needed then go back to try to fine tune the IFR and TSR and bleeds. i made a spreadsheet to calc what PV area i needed to get the whole primary side jetting area with the smaller jets to equal what it was with the baseline jets..

Sounds a lot like my setup except it's a 451 with a really lumpy cam



But the whole point is that I'm having trouble because cruise throttle position, even at 60 mph, is still on the transfer slots
So if I change the IFR, TSR, IAB then I've changed the cruise AFR too! That's why I started with the idle, then the transfer, then the cruise, and finally will deal with the PVCR. And then I can try the secondaries... although it's too darn fast to test full WOT safely on the street.

Anyway it's chilly this morning and I still haven't readjusted the clutch linkage, getting too old to lie on the cold driveway, so no test results yet.
(MAB still .036, #75 jets, 9.5" PV).

Krautrock, did you use a wideband AFR to tune your carb?

no wideband, just reading plugs. if the plugs stay clean and i can get the tune good enough that it drives well in all areas of throttle openings then i'm happy.

Tune the car to run well. Use the O2 meter for information, not final decision making.

Don't lean to heavly on the O2 meter for idle information with a big cam. Remember what it is reading, its not air to fuel ratio.

Yes, I even addressed this several posts ago:



The car ran very well when I put the carb on with the initial tune. The problem is that it was pig rich, wasting gas and darkening plugs.
Hence my struggle to get it leaner on the meter AND running well. The butt-dyno isn't sensitive enough and the nearest strip is 90 miles away...

More (confusing) results.

With #75 main, 9.5" PV, .036 primary MAB, .082 IAB, .031 IFR it may be a little leaner but still runs rich at 60 mph/3000 rpm/15" Hg... I think - the gauge is fluctuating all over the place at that throttle opening - from around 12.2 to 13.8...

(From what I've read, that means the MAB is too big for the jets, and causing air bubbles?)

Light acceleration (holding 10") starting at 60 mph/3000, it climbs to about 14.5. That should be ok.
Power valve only makes a little difference (below 9.5"), doesn't look like even one full AFR. Needs significantly bigger PVCR with the smaller mains than the .059, as I surmised.

What's odd is that if I keep my foot in the primaries but not the secondaries, nearly half throttle (3" vac, up to 4000+ rpm) it gradually got richer as the rpm climbed until I was down around 12.5.
I think that means the large MAB is finally allowing more flow from the mains by 4000?

If I make the mains even smaller (#73 or #74) it will make the air bubbles worse - but smaller on the air bleed will also make the mains come in sooner which I'm trying to prevent!
I have not yet disconnected the secondary link so I can go full WOT on the primaries only.

I don't have enough road to continue 4th gear testing at high revs, high load (4000 is already 80 mph, and in lower gears it accelerates too fast to get a good reading).
What do I do next

Is this right? A smaller MAB delays the onset of boosters? I thought it'd be the other way around, a bigger air leak requiring more airflow to suck fuel through it.

Which brings up another question: Are small main jets and big PVCR affected less by the air bleed at high RPM (than a big jet with a small PVCR)? Or do they behave the same and only the total area is the factor that determines fuel flow?

I'm in over my head here

on the main bleed, as the air speed going into the horn starts to develop pressure. the bleed allows the pressure into the main well and that does the emulsion trick. the more emulsion and pressure, the quicker the fuel mix makes it's way to the booster.
so yes, larger air bleed makes the booster come on earlier.

Thanks! I will try going back to the .033 and .030 MAB and see how it behaves. At least that's the easiest to change
If I can just stay out of the boosters until the transition slot is nearly 100% engaged, my overrich cruise should get better... I hope.

It occurred to me that the AFR might be going richer at higher load/rpm because I only have the primary blades half-open or less, and 3" of vacuum.
So it's acting like a small two-barrel and it's being sucked on really hard

I could disconnect the secondaries and open the primaries to WOT. Bet it wouldn't be as rich at 4000+, and less vacuum too.

Krautrock answered it. Yes, the bigger MAB will make it richer down low and leaner up top. Which is what all Holleys I’ve played with have been lean down low and rich up top. Assbackwards from what it really should be.

Thanks... learn something new every day

Any ideas why my AFR reading is now "bouncing" at cruise?

Dr C what holley 950 do you have? The 830 cfm version? That's what I have and it acts the same way. I finally settled on 70 iab, 28 hsb .081 on the T slot. .031 ifr down low. Its better not perfect. What I need is more idle timing. Without locking out the dis. 34 total 24 initial.

You may be experiencing a case of CarburitisTweakisToomuchus.

Suggest going back to square one as delivered and making sure all is set up correctly. Based on who you bought it from, I would have to guess that the meeting blocks are set up in a pretty standard orientation and the IFR's are down low.

Maybe open up the IFR to .036 or .035 and begin the tuning one step at a time from original config. Idle slots not over exposed and base timing set to around 24-26 depending on what your motor likes at idle and floats set correctly.

Your AFR could be bouncing due to an exhaust leak somewhere, or your O2 sensor is shot/dirty.

Thanks. I agree that there are an awful lot of things to tweak, most of which interact! I may be chasing my tail, but I did start one step at a time

I'll go down the list:

Everything was set up correctly as delivered including IFR moved to low position- the problem is it was pig rich everywhere, worst at the top of the t-slots!
The idle screws are out only slightly over 1 turn with an IFR of .031 (primary) and I left the secondary alone at a pin-gauged .032, although I did plug the baseplate dribble holes.
T-slot exposure is just square on the primaries.
Secondaries are almost closed (butterflies have .100" holes in all four and I run a PCV valve).
Idle timing is 27. It could use even more but I don't want to overdo it while running vacuum advance, street driving and pump gas.
Float levels as described.

Maybe I wasn't clear - the AFR only bounces from low 12 to high 13 when I'm holding a steady 60 mph at 3000 rpm, 15" Hg. At idle it wanders a bit but that's low-rpm misfire from the overlap. During acceleration to 4000 rpm it was quite steady. So I'm thinking "spurts" due to the emulsion, or the large air bleeds?