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carb tuning with wideband O2

15K views 39 replies 11 participants last post by  bhodgdon400 
#1 ·
I'll be installing an air fuel gauge with wideband O2 sensor this afternoon to really try to nail down my idle, part throttle and WOT.

I'm running a Holley HP 750 carb with pretty much out of the box set up.the engine runs pretty strong and smooth overall with no stumbles or hesitations and a good idle but i feel like there's something left on the table in the WOT.


just reaching out to see if any of the great wisdom on this forum can give me any pointers on detail tuning with a wideband from experience and what to look for at different load & throttle positions..


thanks in advance for any advice
 
#2 ·
One thing I've experienced using tuning with a O2 data on carburetor engines is often hitting targeted numbers does not yield the best running car. Sometime being a bit richer is the best running engine (idle, cruise,power, tip in). I believe a big part of this is the fact that the O2 sensor will be providing an average of the cylinders being monitored. One or two cylinders running lead may not show up in the numbers (especially if others are slightly rich) but the lean cylinders can be felt while idling/driving. As a broad brush I'd say a larger cam and/or O.D. amplifies this effect.
 
#3 ·
ok, good to know. thank you. i'll probably plan to err on the side of caution and stay a little more toward the rich end of the spectrum as i understand the limitations of a single sensor down stream on one bank of cylinders.
 
#5 ·
probably so. i've been tuning by feel, smell and reading plugs so far so i'm curious to see where it will be. like i said, the motor runs pretty damn good but i think i might be a little small on jets and leaning out a little bit up high at WOT.

i got the gauge installed and the wiring done yesterday ready to go. I'm taking it to a shop down the street to weld the bung in this afternoon since I'm no welder....lol. should be getting readings on the way home. then i'll drive it some and make notes before i start tearing the carb apart and ultimately ruining all the good i've done over the past year or so...lol

kind of why i was throwing it out there to see if anyone could offer some good pointers on where to make changes and where not to type stuff...
 
#6 ·
I love my AFR gauge. I made a custom mount so I could mount it in my ashtray and am able to close the ashtray and not see it. Lots of differing opinions on what AFR is right for idle, cruising and WOT. It looks like mine is liking to cruise at 14.1 or so with WOT going down into low to mid 12s. I had it a lot leaner which was fine, but my brother who is a Tech at NE Dragway suggested I richen it up some. The tricky part for me has been dialing in the squirters and the accelerator pumps so it will stay at the same when I nail it and hold it until after the accelerator pump shot has run out. for my motor I'm running 81 primary, 89 secondary, and 31 squirters. The tricky thing with mine has also been that someone drilled my butterflies both front and rear. I'm replacing those now. It's not necessary on street motors. It's just a band aid for someone that doesn't know how to tune a Holley IMO.
Running the log on a laptop is also real handy to see what it's doing over a 1/4 mile or a longer cruise. Get edjumacated on what affects each circuit before you touch anything. You'll save yourself a lot of headaches.

20180513_200357 by Dave H, on Flickr
 
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#8 ·
I use a spreadsheet to keep track of changes.
use the PVCR jet for WOT a/f adjustments, your HP should have screw in jets on all fuel circuits.
buy a couple of air bleed kits.

I'm sure I easily have a couple hundred $ in main, air bleeds, idle feed restrictor jets ,accelerator pump discharge nozzles ....

I've attached a spreadsheet that I got started for a friend, make something similar, it's helpful when tuning.
 

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#10 ·
Darin,


thank you for the spreadsheet. that will come in handy. I have some studying to do before i get too deep into this carb. I've only messed with the squirters, pump cams and accel pumps so far to get the lean stumble out of it when i first switched to this carb. now its time to get deeper. I've never played with air bleeds or really anything past main jets, squirters and accel pumps. time to learn what each circuit affects and how to adjust properly.
 
#9 ·
I like 13.5-14.0, cruise is the hardest to get right as they are usually fat, anything in the 13’s or above is good, 12.8-13.2 for WOT

Get the distributor worked out before the carb. My bet is it will be fat at idle and the IMS will need to be turned in under 1 turn from seated. IAB should go bigger if this is the case, go 72-75. Idle air bleeds need large changes.5-10thou.you can drill your own bleeds, just not jets
 
#25 ·
you we're right on the IMS needing to be in under 1 turn to get over 14:1 at idle. I'm still going to do some reading and learning before i start getting into air bleeds or anything internal either. bought the book referenced in another post here and i'm going to hit that cover to cover before i dig too deep. I'm getting more and more of an understanding by the minute.



and to think, I've been messing with these carbs for 25 years or so and had some really strong running cars in that time with only basic understandings.....kind of feels like i was blind and now i see...lol. although i did get around pretty good for a blind guy:nerd:
 
#11 ·
thanks all for the info.

keep the suggestions coming. going to get busy with it on evenings this week and try to get things dialed in this weekend hopefully. based on the quick trip home from the shop that welded the bung in, I'm not too far off but this was a 2 mile drive.

right now I'm idling at right around 13.4:1, cruising a little fat right around 13 and didn't really have a chance to get a good WOT pull yet. gonna get it out tonight to rip into it a bit
 
#12 ·
i kept mine right out in the open. i like the ash tray idea but i'm already using that space for fuel pump shut off switch and registration / insurance storage as the glove box is full of ignition box & coil.

i'm sure the guage is going to drive me nuts eventually and I'll have to re-train my eyes to stay on the road..lol
 

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#13 ·
Looks good. I would have done the same thing, but I'm eventually going to have my car look stock/day two for the most part. She's still a work in progress, but at least I get to enjoy it while I work.
 
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#14 ·
Timely thread. I just installed a bung and added an oxygen sensor and gauge for carb tuning. I'm completely new to carbs (I have a new/stock Holley 4779 750), but wanted one on my 68 so I could learn. So far, it's helped me smooth out my idle, but I've yet to play with cruise and WOT.

Looking forward to learning from this thread.

Daniel
 
#16 ·
#17 ·
From the time I first used the portable ones, I was hooked. 3 circuits to dial in.

Idle
Cruise
Wot

Idle,how far out are the mixture screws, under 1, increase air bleeds by 5 thousand, 75’s usually are middle of the road. How much vacuum do you have at idle in and out of gear and lean cruise driving down the road?

Cruise is driving down the road at 40mph under no load, you are still on the idle circuit. Once you get above 3,500 you are on the mains and can get a good idea if your main jets are close

Wot, get on the freeway and hammer it up to redline trying to drive and
look at the meter at the same time
 
#18 ·
so i got home and screwed around a little last night with the carb and wideband. the best idle mixture i could get from the 4 corner screws was just under 14:1....maybe 13.9:1. any leaner than that and i couldn't keep it running. i figure thats not bad and i know i've got some limitations based on where i installed the O2 sensor, not wanting to drill into my freshly coated headers, i put it right behind the collector on the passenger side.

took it down the road and at cruise speed I'm between 12.5:1 & 13:1. pretty ok, i guess.

the issue now is when i launch from a stop, or stab full throttle from a roll i get a very quick instant lean spike up to and over 16:1. this quickly comes back to the 12.5:1 range when i stay in the gas simulating a drag strip pass....don't tell the authorities. I'm thinking secondary accel pump and shooter increase and i have no lean spike under part throttle acceleration before the secondaries are involved. gonna try bigger shooters in the back first, then possibly a 50cc rear pump.

the other issue I'm having and the main reason i wanted to get into this is that in high gear at WOT the engines is having trouble revving over 5,500 or so. it gets the like lightning but then sits and screams there. before i get too deep into resolving that through the carb, I'm going to add a little more total timing. I'm at 34* degrees right now and that's probably not enough on a bigger cube small block.

i'll report back when i get time to play more and dig deep.
 
#19 ·
Once you adjust your four corner idle, you need to adjust your main idle screw. Your cruise is way rich. You should lean that out IMO. Lean spike is probably from your accelerator pump and nozzles needing attention. I doubt going up to 50s is going to change anything with your problem. It doesn't sound like you need them. What nozzles are you running?
34 degrees total timing would have nothing to do with you not being able to rev over 5,500. Rev limiter? What's your distributor? Any MSD box or the like?
You should take it all back to where the settings were when you were here: "I'm running a Holley HP 750 carb with pretty much out of the box set up.the engine runs pretty strong and smooth overall with no stumbles or hesitations and a good idle but i feel like there's something left on the table in the WOT." Get it there and then the rest will be a lot easier.
 
#21 · (Edited)
"i feel like there's something left on the table in the WOT"

that was the initial issue and still the main target for improvement. pulls super hard in 1st & second as far as i'll let it go, but once in drive at a full WOT pass it seems to lay over around 5,500. doesn't lose power or drop off, just doesn't seem to want to give any more.

the rest of the basic engine set up is as follows;

SBC 406"
6"rods
10.6:1 (with 70cc chambers) forged flat tops
World products sportsman II heads (200 cc intake, 2.05 / 1.6 valves, 70 cc chambers)
hyd roller 234/236 @ 50, .594 lift with 1.6 roller rockers, 112 LSA
victor jr. intake
1 7/8" primary long tube super comp headers
2.5" full exhaust w/ flowmasters

it just feels like it should be able to pull more in high gear. i may have to look at the converter as well.
 
#27 ·
My AEM AFR was the best money I spent. Now you can tune each circuit and know what effect each change had. My carb was rich which was killing power, night and day difference. Of course if yours is already close then you may not notice as much but should at least get better MPG and more life out of your spark plugs.

1. Before changing anything record all your current settings:
Primary Jet:
Secondary Jet:
Power Valve:
Power Valve Restrictors:
Idle Feed Restrictors:
Idle Air Bleed:
Highspeed Air Bleed:
Emulsion Jets:
Pri squirter:
Sec Squirter:

2. The following needs to be set correctly before you begin:
- Float Level (carb specific some models require fuel at the bottom of window others the middle of the window)
- Consistent Fuel pressure (even at WOT)
- Transfer slot exposure (set it to a small square and then don't use your idle screw to change your idle. This can cause too much transfer slot to be exposed leading to a stumble. Adjust the secondary blades instead.)
- Timing. I assume the goal is to have a lean idle and cruise. Lean mixtures take longer to burn therefore more initial timing. This is where a vacuum advance distributor hooked to full manifold vacuum comes in. It advances your timing during idle and cruise and will make your motor idle at a higher rpm (so your blades can stay within spec).
- No play in the accelerator pump. As soon as the throttle moves at all it should squirt.

3. Only make one change at a time. All tests drive results should be based on a fully warmed up engine (cold engines run lean) and a decent amount of driving.

4. Hook up a long hose and vacuum gauge to full manifold vacuum so you can look at it while driving. This will help you choose your Power valve and allow you to tell what effect the power valve and the power valve restrictors have on your AFR. Cruise vacuum can be much higher than idle vacuum so it doesn't really make sense to pick your power valve based on idle vacuum. "But Holley says to" Holley isn't going to tell you to hook up a vacuum gauge and drive around looking at it. Pick a number that is about half your light cruise vacuum reading. The power valve is going to allow you to tune a lean cruise and richen up when you get on the throttle so you still make good power.

5. Learn about each circuit and pay attention when you drive to throttle position, load, vacuum, and RPM to determine what circuits are in play so you know what changes need to be made.

Each circuit builds on the other so start tuning in this order so you aren't chasing your tail:

Idle circuit: (Throttle closed) Mostly Idle mixture screws (all 4 turned out the same amount) Also Idle air bleed and Idle feed restrictor
Transfer circuit: (Throttle slightly opened) Idle air bleed and Idle feed restrictor
Main circuit: (throttle 1/4 open or more) Primary jets
Main circuit: (throttle 2/3 open or more) Secondary jets
Power valve circuit (when you open throttle fast enough to cause the vacuum to drop, towing, going up steep hill) Power valve vacuum rating, some have bigger or more doors to flow more fuel, Power valve restrictors, work like jets to control the amount of fuel the power valve provides.
Accel Pump circuit (when you increase throttle position) Accelerator pump and squirters

6. For gas 14.7 is stoich (complete burn of the fuel). Higher is leaner. Lower is richer

Too rich kills power, fouls plugs, wastes gas, smells bad
Too lean melts piston, detonation (but this is under load or WOT. Idle, cruise, and decel can all be lean without issue)

You aren't targeting just one AFR and there isn't one perfect number for every motor. For example a lumpy cam might want a richer idle. Ideally you want idle and cruise to be lean and the further you open the throttle the more rich it gets. When you get below 12 you are probably going to start losing power.

Rough outline:
Idle-2500 rpm 14-15
2500-4500 rpm smoothly transition to 13.5
4500-7000 rpm smoothly transition to 12.5

7. Main jets - Moves the AFR up or down through the entire RPM range. Change up or down 2 sizes at a time.

Air bleeds change the AFR curve. As the rpm increases it gets richer. Change up or down 4 sizes at a time. (since there is more air then fuel it takes bigger changes to see a difference) Air bleeds work the opposite of jets. Bigger air bleeds make it leaner, Bigger jets make it richer.

Emulsion jets also tune the AFR curve.
 
#28 ·
The next time you take the carb off, look at how far the throttle plates are open and even them out, the slot showing should be square and not a rectangle

The lean spike on take off is the meter trying to react to the change from idle to wot
 
#35 ·
I've been looking at that system for a while now. it's not completely off the table. just think i'm going to go one more year carburated and see how good i can get it. i'd really be interested to hear your thoughts and experience on install and set up of the pro-flo 4 and how you like it driving.
 
#34 ·
Buy the 10 packs of air bleeds and drill them yourself, one set at .075 and the other at .080. Or just drill the .075 and then if you neeed more, go to .080.

If your IMS are not out 1 1/2 turns, your air bleeds are to small or your IFS are to big, which they always are with these carbs. Air bleeds should be your first test
 
#36 ·
FWIW... There can always be a small problem with your carb that will throw all the advice out the window. I "wasted" most of my day on Saturday. Although, is working on your 69 really in anyway wasting your day? My 950 carb had two rear float bowl screws that were stripped and started to leak. I figured I'd swap out one of my 850 Holley's temporarily while I got Helicoils to fix it. I first rebuilt one 850 since it's been sitting on a shelf after running ethanol fuel and set it up the way it should have run pretty well. Wouldn't idle. Screwed with it for a while and decided that I'd just use my other one that I've used before. Same situation. I rebuilt it with new jets 79/85 to start, new 6.5 PV for the front, blocked the rear. New accelerator pumps (30/50) and 31 nozzles. 1 1/2 turns initial for the four corner idle screws. Squared the transition slot. Ran ridiculously lean. Finally put the car away and said I'd just wait and fix the 950. I suspect I need to hot tank the other two carbs, but I did blow out each passage with carb cleaner.
 
#37 ·
I've had many a "wasted / not so wasted" day chasing my tail as well. which brings me to my Saturday.......as stated earlier, i intended to start getting deep into the carb tuning this weekend. well, like the responsible sane person i pretend to be sometimes, i decided to do the right thing and re-check / confirm timing before i got into the carb tuning.



for reference, I'm running an MSD 8361 (i think) distributor , it's the pro billet dealy with vacuum advnace, an adjustable vacuum advance can hooked to manifold vacuum with a limiter plate for 10* additional advance, an MSD 6AL box and blaster 2 coil.



the begining set up in the distributor was the 2 lightest silver springs to bring all advance in by 2,000 with the green limiter bushing to limit mechanical advance to 23*. initial set at 12*


disconnected the vacuum can, plugged the port on carb base and hooked my light up. once at full operating temp (175*) i read my stated 12* initial advance.



this is where things get weird.


with the light on the balancer, i bring the RPM's up to watch the mechanical advance. as the rpm's reached 2,500 or so the advance quickly ran up to right at 30* then just as quickly settled back down to 20* and remained there regardless of how much rpm input i added. maybe I'm missing something but shouldn't the advance get to around 34-36* with this set up and stay there with added rpm's?


so i confirmed what i just listed above a couple times and check with three different timing lights. (2 dial back style, 1 fixed style) got the same reading each time.



at this point i was figuring i had the wrong mechanical stop bushing in the dizzy and wasn't allowing it to get the full advance timing. I pulled the distributor, found the green bushing in there that i had in my notes. i replaced it with the silver bushing that should have added 25* or so and replaced on of the springs with the light blue spring that would make it come in a little higher, around 2,500.


replaced distributor, reset initial to 12*, ran it up and got the exact same readings as before. pegged to 30* then quickly came down and settled at 20* no matter of added rpm even with the smaller stop bushing. goes right back to 12* at idle.


i tried adding initial timing up to about 18-20* then brought the rpm's up again. same thing, initial would be right where i set it, mechanical advance spikes to 30* then drops back to 20* and stays there.



what am i missing here? should i start a new thread with this? called MSD tech line but they are closed on saturdays. may try another call today if someone here can not add some clarity.
 
#38 ·
needless to say, after my timing woes, i didn't get too deep into the carb tuning. i did get the idle mixture a little better to right around 14. -14.5 bouncing around there. the 4 idle mixture screws are about 1 1/4 turns out and I'm idling at about 700 RPM so that's not too bad.

theni ran out of my allotted car time and had to go be husband again...
 
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