Tuning your carb with an O2 sensor

[JeepFreak]

I was thinking of using an O2 sensor to tune my Holley and welded the bung in for the O2 sensor in yesterday. Well early this morning I was very suprised with the readings I got from the DVM. At idle was getting reading of around 700mv which explains why she doesn't idle all that great. I adjusted it down to around 450mv which would put it just a little over a perfect ratio.(will more than likely bump this up some) I then went on the road with DVM in hand to get some readings there. Well I was very very suprised to find that I was running very rich(at at steady 50mph on a flat road) with a reading of over 1v so it was back to the garage to change the primary jets. After many trips I dropped down 5 jet sizes to finally get a good 600 mv reading which seems well suited for my motor. I was very suprised at just how well this method works on a carb! For those of you thinking that I am just really smart well think again because I found a really good site that told me everything I needed to know about tuning any carb with an O2 sensor. I am no dumb dumb when it comes to mechanics(worked as one for 8 years) but carb tuning has always been an area that I wanted to get better at. For those of you that think you know it all please check out the site below because there is lots of good info here. I e-mailed the guy who runs the site and he was quick with return e-mails and helped me alot. This method can be used on any carb so don't think that because he talks about a Holley that it doesn't apply to you because it does. Well seems that O2 sensors aren't just for you FI guy's anymore!

Carburetor Tuning the Scientific Way http://www.bob2000.com/carb.htm

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JeepFreak

[Veepster]

congrats Jeep Freak!!! I used an O2 with my carb for years before I got into EFI......you are right though, it is WAY simple to do!

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Peace.............BartG

the Green Flash!
360 2v, TH400 QT with low
4" skyjacker Rancho 9000's
33x12.50 BFG KO's 8" American Racing Baja Rims
Thorley headers, 3" exhaust, Dyno Max
Infiniti power leather seats
50%Luxury Car, 50%Tractor
http://www.teamgodspeed.com

[JeepFreak]

Who would have thought that you could apply new technology to a system thats 100 years old! http://www.ifsja.org/ubb/smile.gif Veep have you seen this site before? Just when you think you know it all you learn something new and realize how little you knew.

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JeepFreak

[Veepster]

I had not seen it before....but he is 100% correct!.....and very well said!......he explained the different circuits very well...I wish i could write that well!....good luck with your O2...it is really that simple!.....be careful not to make the carb too lean......be safe and error on the rich side!

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Peace.............BartG

the Green Flash!
360 2v, TH400 QT with low
4" skyjacker Rancho 9000's
33x12.50 BFG KO's 8" American Racing Baja Rims
Thorley headers, 3" exhaust, Dyno Max
Infiniti power leather seats
50%Luxury Car, 50%Tractor
http://www.teamgodspeed.com

[nograin]

Jeepfreak, I confess I did see the site before (and went back to it when the O2/FI thead was going hot and heavy). Always learning new stuff like you guys said.

Where did you put the bung in? I was thinking It might be better to go with a heated O2 sensor and go in after the two sides join. Sounds like you just went with a single wire. Also, are you leaving the sensor in, or did you plug it up wit something?

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360 auto select
body by beer (PO)
carries wood inside
no "wood" outside

[Veepster]

as I recall...if you want to remove the O2..a sparkplug(don't know which size) will fit perfectly!

------------------
Peace.............BartG

the Green Flash!
360 2v, TH400 QT with low
4" skyjacker Rancho 9000's
33x12.50 BFG KO's 8" American Racing Baja Rims
Thorley headers, 3" exhaust, Dyno Max
Infiniti power leather seats
50%Luxury Car, 50%Tractor
http://www.teamgodspeed.com

[JeepFreak]

I welded mine in on the y pipe just below the manifold. Followed the instructions exactly the way he had them and had no problems. Hardest part was getting the y pipe out of there. I was thinking of getting the lean rich meter and mounting it in the dash somewhere just so I have an idea of what the carb is doing. Would be nice to have a gauge give you an idea of when its time to do a rebuild. Well that might be streching it a bit but still I would like to see whats going on under there.

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JeepFreak

[RudyC]

Wow, good find JF.
This makes it day and night when dealing with these Holleys.
FYI O2 sensors are bountiful at junkyards.
(and very small, err cheap http://www.ifsja.org/ubb/biggrin.gif)
And face it, that as a FSJ owner you should already have a vlotmeter and a vacuume guage.
I have a spare heat riser valve that i can drill and tap to take the sensor http://www.ifsja.org/ubb/biggrin.gif.

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"THE GREAT SANTINI"
1979 WAGONEER, 360,
Holley carb.
Stock 4bbl Intake,
TFI CONVERSION, TH400, Q-TRAC. 3" ADD A LEAFS, 31" MUD T/A'S (PEP BOYS CLONES).

http://home.earthlink.net/~zzjoecool/index.html

[RustyJeep]

Did someone mention the words, "Five finger discount"?

------------------
RustyJeep
-----------
'90 GW 360
(for sale)

Project "P.O.S."
'82 J-10 258
Bone Stock...For Now
Can anyone say, "Fred Flintstone" here?

'85 AMC Eagle
258 cubic inches of pure...umm...power. Yeah that's it.
Lifted Eagle comes to mind.

[nograin]

http://www.ifsja.org/ubb/cool.gif So you didn't worry about trying to measure both sides? 'cause it should be pretty balanced (not something wacky like a 3 2bbl set up).

Anyone out there good with electronics??
What would it take to make an indicator light like JF is talking about. I'm thinking 2 LEDs, a power source and something to switch on and off at say 550 mV....

[JeepFreak]

I thought about a salvage yard one but sometimes they are tough to get out and I wanted to just get a new one to rule out a bad one.

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JeepFreak

[JeepFreak]

On that site I gave he gives a part number for one of those lights from Summit Racing. I put my O2 on the drivers side, no real reason why just did. I can't think of a reason why you would really need two sensors. A carb will give the same amount of fuel to all cylinders(as long as there are no problems). A carb is a supply and demand type system. More than likely if one side is right on the other will be as well.

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JeepFreak

[nograin]

My problem JF is my early learning came from NHRA racing guys. Real technical perfectionists. Make me look casual. But to win in class racing thats what you gots to do.

To answer your question. There can be a difference from the left to right bank. My guess is it has to do with the firing order, and can be effected by manifold carb combinations. I would bet that changes in temperature from summer to winter would effect the fuel air ratio more signifcantly than the difference from the left to right bank on any street driven v-8, and I don't know any non-racer who changes their jets on a seasonal basis!

So yes, NHRA class racers, and some bracket racers do stagger jet their 4 barrels if needed. The 6 pack, or tri-power set ups almost always need it.

Hmmm, you must be right about that indicator, he did mention that all O2 sensors have the same output.

[JeepFreak]

I change my jets seasonally. In fact I went ahead and bumped mine back up for a richer condition because it's been getting really cold in the mornings. I can see the use for two sensors on the applications you discribed but for the normal street ride with a single carb I think two might be over kill. Now on the other hand some intakes split the left and right runners and I could see it being usefull there for adjusting idle and maybe some other adjustments.

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JeepFreak

[graham]

<BLOCKQUOTE>quote:</font><HR>Originally posted by JeepFreak:
I was thinking of using an O2 sensor to tune my Holley and welded the bung in for the O2 sensor in yesterday. Well early this morning I was very suprised with the readings I got from the DVM. At idle was getting reading of around 700mv which explains why she doesn't idle all that great. I adjusted it down to around 450mv which would put it just a little over a perfect ratio.(will more than likely bump this up some) I then went on the road with DVM in hand to get some readings there. Well I was very very suprised to find that I was running very rich(at at steady 50mph on a flat road) with a reading of over 1v so it was back to the garage to change the primary jets. After many trips I dropped down 5 jet sizes to finally get a good 600 mv reading which seems well suited for my motor. I was very suprised at just how well this method works on a carb! For those of you thinking that I am just really smart well think again because I found a really good site that told me everything I needed to know about tuning any carb with an O2 sensor. I am no dumb dumb when it comes to mechanics(worked as one for 8 years) but carb tuning has always been an area that I wanted to get better at. For those of you that think you know it all please check out the site below because there is lots of good info here. I e-mailed the guy who runs the site and he was quick with return e-mails and helped me alot. This method can be used on any carb so don't think that because he talks about a Holley that it doesn't apply to you because it does. Well seems that O2 sensors aren't just for you FI guy's anymore!

Carburetor Tuning the Scientific Way http://www.bob2000.com/carb.htm
<HR></BLOCKQUOTE>

JeepFreak,
I've done the same thing as you and fitted an o2 sensor. 2150 feeding a 360, reading 380-400mv at idle but jumps to 950+ at 50mph. As you've been there and done it, Can you help with a few Q's.
Do you have the same carb/motor combo as I do?
Primary Jets... What size did you start from and what size are you using now?
Any other mods to the carby?
TIA.

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Graham
http://www.geocities.com/ozbearhunter/Sig_Pic.jpg
'BEAR HUNTERS" HOME PAGE (http://www.geocities.com/ozbearhunter/)
'82 Cherokee Sportsman II,360, auto. As it rolled of the Show Room floor.
Except for;LPG conversion, On board air, plus O2 sensor.

[Ralph]

Okay, I'm just going to weigh in my nickel's worth in this old thread.

Carb tuning is not the same as tuning EFI, most basically because EFI takes readings from the lambda/O2 sensor (among other sensors) and electronically adjusts the duration that the injectors are opened.

In contrast, carbs work on vacuum and meter fuel according to airflow: mechanically, according to speed and volume of the airflow.

So an O2 sensor should not be relied on as the exclusive tool when tuning a carb. Learn to use a vacuum kit (the one Sears sells has excellent instructions in the box for carb tuning). You'll find that your O2 sensor and A/F meter will give you much more favorable readings if you tune the carb by vacuum settings.

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J ust
E mpty
E very
P ocket

[JEEPRZ]

FYI, you dont need a fancy O2 sensor bung for this...go to the HELP! section at your local auto parts store, and get a 18mm spark plug non-fouler. Cut the end off, and weld it on. Super cheap and easy. http://www.ifsja.org/ubb/biggrin.gif

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Dave
83 GW
360/727/229
No longer a tow rig...now its buildup time
3"lift, 31s, 3.31s, edlebrock intake, cam, TBI, custom DIY exh, rebuilt trans, shift kit and factory aux cooler,modified 229 (no viscous coupling, *improved* vacuum system), lotsa squeaks, leaks, and rattles

[nograin]

I don't think you are that far off. I just dynotuned my 'cuda and its kindof like Ralph says. We took it two jet sizes leaner than the best hp reading and didn't hurt hp, but messed up mid range drivability completely. Even two jet sizes leaner we were richer than 14:1 through most of the range. At WOT I would gues 13.5 to 13.8 is good. According to the chart at the BOB2000 page, 900 mv is in that ballpark.

I have a vacuum gage in my dash (stock!) and it is not very helpful for road tuning, but great for getting the best idle. It is also great for diagnosing problems, both at speed and at idle.

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360 727auto-trany, NP229
body by beer (PO) :(
carries wood inside
no "wood" outside
http://users.erols.com/mathewg/85gwsmll.jpg
My other car is a fish

[Veepster]

hey Ralph...you are right on the carb using vacuum for function but......a/f ratio is a/f ratio no matter what the method of delivering this fuel.........the O2 compares the oxygen content in the exhaust to the ambient oxygen content....so deliver method is a mute point......

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Peace.............BartG

the Green Flash!
360ci
Custom 4v TBI
Edelbrock Performer Manifold
TH400, QT with low
4" skyjacker suspension lift, 3" body lift Rancho 9000's
33x12.50 BFG KO's 8" American Racing Baja Rims
Thorley headers, 3" exhaust, Dyno Max
Infiniti power leather seats
Custom billet shift knob by Millerluck
50%Luxury Car, 50%Tractor
http://www.teamgodspeed.com

[JeepFreak]

WOW was suprised when I saw this old thread brought back to life again! I am running the stock 4V and Holley carb on the motor in question. I merely did this as a challenge to see if I could get it to work. Ralph is right on with the vacuum gauge. Since a carb works off of vacuum this is the best way to tune them.

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JeepFreak

[Matt W]

JF, you got a choke on your holley? i removed my choke airvalve but kept the junk on the side of the carb. it idles fairly ruff, but i got an edbrk performer cam,intake, trw 9.6 pistons and no vac advance hooked up...it started pinging at low rpm/low manifold vacuum,(ie going up a steep hill at 30 to 45 mph), i also have to run 91 octane at 4400 ft elevation. is that normal, you think? oh, i got rid of the choke cuz 1, it has an "aerobic short" in the hot wire, and 2 it started like a ***** no matter what i set it at in summer(70 to 100 degrees) and winter, (-30 to 45 degrees). Since pulling it, it starts and runs better but still not too smoothly.

[RudyC]

Wow Matt W. 9.6 to 1 pistons. Pretty stiff CR. I dunno about the Edelbroke cam either. what is your timing set at? A few cams we installed needed like 10 to 14 deg advance before they would even Idle.
What carb you got? settings?

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79 WAGONEER (SOLD IT, BUT WILL FIND A LESS RUSTY ONE LATER)
79 DODGE, MY LIL RED TRUCK. 318, 4BBL HOLLEY,TF727.
pictures at: http://albums.photopoint.com/j/AlbumIndex?u=1500380&a=11321052

[This message has been edited by RudyC (edited May 08, 2001).]

[JeepFreak]

Matt I'm running about the same compression as you are and I can run 87 most of the time. I had my block decked to get my compression up though. I also run the Edelbrock intake manifold, cam, and carb. I too can not run vacuum advance. Why I'm not quite sure. I need to change advance springs in the distributor and fool with the timing a little more to see if I can get it nailed down. I have a bad flat spot at 1,500 rpm which is probably due to the timing and no advance.

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JeepFreak

[Matt W]

MAN oh man! when i rebuilt this 401 i was thinkin' nothin' but VROOM! I ordered the highest comp pistons trw made in a forged 401 size. w/ my combustion chambers being the larger cc ones, that equaled 9.6:1 the pistons in the previous 401 i had were 8.7:1, my dad rebuilt that one in '83 or 4 or something. youcan get like 10.6:1 if you use the smaller combustion chamber heads...pre '71. TRW also makes 390 pistons up to 13 to 1, which, if you did some milling off the domes, you could fit in a 401...they're the same bore! Any way, i just checked my inital timing and it was around +5 i moved it to +10 and it smoothed out considerably. it idles at 15"of hg in park @ 700rpm. the carb is a reverse idle mixture 600 cfm 4150...i think thats the right model number. any way, its got a primary AND secondary metering block w/ jets. Primary's are 60 and secondaries are 67. i had it jetted leaner, but there was no top-end power. the power valve is a 8.5 the shooter is a 25, and i'm using the brown pump cam. all this I'm reading from a notebook where i wrote it at the time of the carb rebuild some months ago, i don't think i've changed it since then. no vacuum advance at all! it's a flawed idea on an automatic, if you could grab a gear every time you went up a hill,and your rpm could match your manifold/ported vacuum, then i would maybe run it, but i was getting my advance at low manifold/ high ported vacuum at low rpm up hills at 35 to 45 mph. "ping ping ping" BAD!
Tires are 32" and gearing is 3.73:1. Cam is set to 0 degrees advance or retard from crank, just put on strait. i don't know, maybe those 5 or so degrees i added this afternoon will help my mileage? prolly couldn't hurt. oh yeah, the cam is out of the previous 401 motor, its prolly got close to 80k miles on it!20k of it on this motor. If i would have been really experianced at engine rebuilding, i would have realized that by having such high compression, i would have to run rich/little advance/ and high octane, i might've gone w/ 8.7:1 pistons, but stock is 8.23:1 and i wanted POWER! http://www.ifsja.org/ubb/biggrin.gif ah well, I'm thinkin' of building a 390 stroker which is basically a 360 block w/ 401 crank and rods, using 360 pistons, i got everything to do it except the machine shop and the amc guys to tell me exactly what is required to run the 401 crank in a 360...it fits in the journals, i just dont know if i'll have to notch the bottom edges of the cylinders to clear the rotating assembly. sounds cool to me though. http://www.ifsja.org/ubb/biggrin.gif less reciprocating mass w/ greater stroke and stronger crank and rods...can't go wrong!