Tuning carb by Air/Fuel readings

[GearAddict]

Hey Guys, I've posted some of this is other sections on the forum without much luck. Hoping the race crowd, might have some knowledge they can lay on me.

I bought and installed an AEM wideband UEGO gauge in hopes of dialing in my Holley 670 Vac Sec carb.

Any help on interpreting results, test parameters and adjustments are appreciated.

I temporarily wired the gauge inside the car so I could see it while driving under load.

I took her out for a drive last week and tried to log some results. I'm thinking I will take a video and post later this week, as the gauge is constantly moving, so at best at the moment, I can make general comments on what I saw.

1. Idle - 13.5-14.4 @ 1K

2. Short burst (2-3sec), WOT, 2nd gear, 3rd gear.. quick blip to 11:1 then hangs in the 12.5:1 - 13:1 range for the remainder of the pull. Pulled hard, with slight stumble observed on one of the pulls... I have a very light spring on the Vac Sec... maybe went too light?

3. Part Throttle cruise, held steady in the 2K-2.5K RPM range 3rd & 4th gear, Looks to run lean between 15.0 - 16.0

My initial, uneducated thought is that Idle and WOT is good? but the part throttle lean needs to be solved?

If so... can someone help me with what the best way to address is? Jetting? Is this a idle transition circuit issue?

Also, if someone has suggestion on specific conditions to record data (aside from the above steps).


---Update--

I took the car out yesterday for a short drive and noticed it had quite a few spots where it went lean. It seemed sporadic but the duration of the lean was long enough that I don't think they were caused by misfires

I bought a set of 71 Jets so I thought I might change out my 65/68's to 68/71 and see if that helps.

I'm worried that the low vacuum is causing me to expose too much of the idle transfer slot and that I might have to drill holes in the butterflies to compensate, but I'd rather not, since I'm planning to swap the cam out for something with better idle/vacuum characteristics than the MP 509.

 

[mentor70]

a carb swap would be easier than a cam swap...IMO.

i had a similar issue with my 496...ended up selling the car due to frustration, and lack of funds.

this time around i bought the Holley HP ultra, so it has all the compensation capabilities.

BTW, the power valve size needs to be half of whatever manifold vacuum you have.

 

[cryplydog]

GearAddict, cams with a lot of duration like the purples run like a bastard but are hard to tune at low RPMs with regular carbs.you are going to need a carb that is designed to tune at between 240 to 260 degrees duration @ .050 . there are several good carbs brands available but I like and have had great luck with Demon but have used most brands. You didn't say what engine or performance level it was at so I would recommend asking tech what they would recommend. Good luck

 

[GearAddict]

Thanks guys.. .yeah, realize I left out specs.

This was an inherited motor from the PO. General specs...

383, MP 509 cam, Edlebrock RPM manifold, headers, MSD Ign, Holley 670 SA, Hand ported 516 heads.

4 speed, Dana 3.54 gears

Vac = 7 at idle, 1.1K RPM, Timing 20 initial, all in 2500, 38 total

Holley is set up with 3.5PV, 65P and 68S Jetting.

I've been looking at a Quickfuel 750 DP, but was holding off for the moment, because I think the motor is overcammed and was intending to go to a shorter duration grind with an aggressive ramp (Hughes mopar grinds maybe?). Didn't want to buy a carb and then need to purchase again.

For the moment, was just thinking I would learn a little more about tuning on this carb, get some experience in A/F (and real life changes) from what adjustments I had available. Then after swapping cam, buy new carb and apply that knowledge.

One thing I am realizing is that adjustable air bleeds and a 4 corner idle circuit would really help in dialing it in... but for the moment that's not an option, so I'm looking for any advice you can give to help with my current set-up.

 

[evilmopar]

I have the same issiue here, runs lean on part throttle and i have quite low vacuum on part throttle around 16-20inHg @60mph on flat road. I solved it by giving a bit more fuel on the idle screws so on idle i run about 12 AFR.. that works in my case cause a carb swap isn't so easy in my case cause i run a turbo setup. So i'll change cam this winter to get rid of the vacuum ;-)

 

[Hemirunner]

With the closed chamber heads, 38 may be a couple of degrees past max hp but that depends. Adding some timing at idle and low rpm will make that cam much easier to tolerate. I'd run 30 at idle and limit the mexhanical to give you another 6 by 2500 rpm or so assuming CR and fuel quality allow it, then run a start retard to allow it to crank without kicking.

 

[GearAddict]

I Just realized that I have a spare throttle body. I may just drill the secondaries and see if that allows me to expose less of the transfer slot.. hopefully this will help the cruise A/F mixture.

- - - Updated - - -

With the closed chamber heads, 38 may be a couple of degrees past max hp but that depends. Adding some timing at idle and low rpm will make that cam much easier to tolerate. I'd run 30 at idle and limit the mexhanical to give you another 6 by 2500 rpm or so assuming CR and fuel quality allow it, then run a start retard to allow it to crank without kicking.

I'm running an MSD dist, the stock bushing will only pull the advance down to 18 over initial... I believe someone makes additional bushings to bring it lower, I'll have to check on that.

I have been looking at an MSD retard unit, looks like that would work if I can find the above bushings.

 

[Hemirunner]

If you're running a pro billet, just pull the shaft, twist 180 and reinsert to lock it out. Then just run full timing and the start retard.

 

[GearAddict]

Thanks, but unfortunately its a Ready to Run Dist.. Although I think there is a way to lock that down as well.

 

[Bigal75]

Hey Guys, I've posted some of this is other sections on the forum without much luck. Hoping the race crowd, might have some knowledge they can lay on me.

I bought and installed an AEM wideband UEGO gauge in hopes of dialing in my Holley 670 Vac Sec carb.

Any help on interpreting results, test parameters and adjustments are appreciated.

I temporarily wired the gauge inside the car so I could see it while driving under load.

I took her out for a drive last week and tried to log some results. I'm thinking I will take a video and post later this week, as the gauge is constantly moving, so at best at the moment, I can make general comments on what I saw.

1. Idle - 13.5-14.4 @ 1K

2. Short burst (2-3sec), WOT, 2nd gear, 3rd gear.. quick blip to 11:1 then hangs in the 12.5:1 - 13:1 range for the remainder of the pull. Pulled hard, with slight stumble observed on one of the pulls... I have a very light spring on the Vac Sec... maybe went too light?

3. Part Throttle cruise, held steady in the 2K-2.5K RPM range 3rd & 4th gear, Looks to run lean between 15.0 - 16.0

My initial, uneducated thought is that Idle and WOT is good? but the part throttle lean needs to be solved?

If so... can someone help me with what the best way to address is? Jetting? Is this a idle transition circuit issue?

Also, if someone has suggestion on specific conditions to record data (aside from the above steps).


---Update--

I took the car out yesterday for a short drive and noticed it had quite a few spots where it went lean. It seemed sporadic but the duration of the lean was long enough that I don't think they were caused by misfires

I bought a set of 71 Jets so I thought I might change out my 65/68's to 68/71 and see if that helps.

I'm worried that the low vacuum is causing me to expose too much of the idle transfer slot and that I might have to drill holes in the butterflies to compensate, but I'd rather not, since I'm planning to swap the cam out for something with better idle/vacuum characteristics than the MP 509.

I would start by adding some main jet. Is it surging?

 

[coloradodave]

You can buy those bushings here: http://www.4secondsflat.com/MSD Distributor Tuning.htm

 

[GearAddict]

I would start by adding some main jet. Is it surging?

Maybe a touch, but not severe or strongly noticeable.

 

[Bigal75]

Maybe a touch, but not severe or strongly noticeable.

Try and increase your main jet by 2 to see if that helps - usually the main circuit is active around 25-3000rpm (driving) . You could always plug the powervalve and test this on a flat straight road. Another thing you could do is richen up the idle to 12-13 and see if your part throttle improves, that would show that the issue is in the idle circuit.

Make sure your timing is good first and dont drill the throttle plates - square off your transfer slot (0.020) on the primary side and try adjusting your idle on the secondary side -you will probably have to take off the carb for this.

 

[GearAddict]

Thanks, I'll try that. I have a set of jet 3 up in primary and secondary, I'll try those and then dial back if it's too rich.

I'll adjust transfer slot on the primary and sec side while I am doing the jets and report back.

 

[Bigal75]

Thanks, I'll try that. I have a set of jet 3 up in primary and secondary, I'll try those and then dial back if it's too rich.

I'll adjust transfer slot on the primary and sec side while I am doing the jets and report back.

make sure the transfer slot is showing on the primary side, use the secondary side to adjust idle (little screw thats accessible with carb upside down) no need to jet up the secondaries.

 

[GearAddict]

I made several adjustments yesterday and then ran some tests.

1. Reset idle to ensure only .020" of the transfer slot exposed. (I had to back off the fast idle cam screw to achieve this, turned out it was engaging the cam even at full open choke position).
2. Opened Secondary butterflies, hard to quantify, but I tried to do it to match the primaries, based on how much light I could see coming through
3. Changed Jetting on Primary from 65 to 68.
4. Reset Idle mixture screws to 1.5 out.
5. Reset timing to 18 as I have the mechanical adv limited to +18 bushing.. 18 initial 36 total (this was actually done before I pulled the carb off).

During initial run, Idle RPM was too low to maintain itself (800ish). Since I backed off the fast idle to get the transfer slot down to .020, it was no longer helping keep the RPM's up. I stayed on the gas until it warmed up, then gave incremental increases to the idle speed screw to get it stable at 1K RPM.

During all of this the mixture varied but was on the very lean side (16-17:1). I tried opening the mixture screws to compensate, but was not able to get any better than 15.7:1 (varied from 15-16.2)
The idle mixture screws had no effect on RPM.

Vaccum sat at about 5 through most of the testing.

Now, I raised the RPM to 1,150ish and all of a sudden I'm running 13.6:1 and vacuum at 7-8.

I took her out for a drive and she ran ok.. was right in the 13-14:1 range at idle and cruise and would go rich (11:1) for a second when I hit the gas and then return to 13:1 for the rest of the pull (short burst of accel for 2-3 seconds from 2K RPM cruise in 3rd, up to 5K).

Maybe this is ok and 1,200 RPM idle is where it needs to live, but I'm a little baffled why 1,200 RPMs is giving me 13-14:1 and 1,000 RPMs is giving me 16-17:1?

thoughts?

 

[rumblefish360]

It sounds like the jetting may be a bit off and the extra blade opening is allowing more air in. On the opening amount, you maybe getting into the main circuit.
RPM wise, I myself have never had a engine (even a ratherly heavy hitting cam size inside) want to idle over 1,000.

 

[Dibbons]

Just a comment on idle speed: my '65 273 with full competition Isky 1012B camshaft idled "smoothly" at 1200 RPM (advertised 312 duration and .580 lift)

 

[Jcharger]

I made several adjustments yesterday and then ran some tests.

Maybe this is ok and 1,200 RPM idle is where it needs to live, but I'm a little baffled why 1,200 RPMs is giving me 13-14:1 and 1,000 RPMs is giving me 16-17:1?

thoughts?

I hope I have read your thread correctly. I think the reason for this is: when you open the butterflies to raise the idle that you could have exposed the transfer slots which then control the mixture rather than the idle mixture screws controlling the mixture.

I could be wrong, but I will explain what I believe is happening.

Main Circuit: The main circuit (high speed to WOT) is controlled by the main jets. It does not seem like you are that far off from the 12.5-13.2 a/f that I read is best. So those look OK and I would leave those alone for now.

Accelerator pump: The accelerator pump and pump cam also cover up the spot where air comes in as the throttle blades are opened and before there is enough air velocity to draw fuel through the boosters and jets. It doesn't sound like this is the issue either, so I would leave that alone.

Idle and Cruise: This is where I would focus (as your are). When the transfer slots are barely exposed, the fuel is redirected through to a lower port that is metered by the idle mixture screws. This lets one close down the idle mixture screws and the amount of fuel decreases, open them up and it increases. When the transfer slots are fully exposed, the fuel is pulled through them instead.

My guess is that you adjust only the primaries and these are the ones with the idle mixture screws. The idle circuit at cruise (above 1300-1500 RPM) is controlled by the transfer slots (and I think to some extent the idle mixture screw settings). The whole thing can be adjusted by changing the idle feed restrictor or idle air bleed (but I do not believe these can be changed on your carburetor except by drilling - I also would do some more checking and testing before trying anything that was not reversible), and the low RPM vs mid RPM idle should be controllable with the idle mixture screws.​

The fact that the idle mixture screws cannot bring the mixture below 15-16-17 is something I do not know the answer to. It would seem to me that this means the mixture screws cannot be opened enough to deliver enough fuel or that there is a vacuum leak somewhere that is letting in air. Are all the ports blocked, including the PCV valve?

My guess is that you may be able to get it to idle lower if you can get a richer idle mixture (even as low as 12.9) (I believe the 509 is a big cam for a 383 - but know people run them and understand the idle is higher than with a smaller cam.)

I hope this helps.

 

[Dennis H]

I am here to learn. After full warm up it idles on its own right? 800 should be enough without nursing the throttle.