Tuning with A/F meter

[randlej]

Some questions on setting jetting for cruise and WOT. I just hooked up my new Innovate LM2 today and have the following numbers on a 351W with 650 double pumper running 68 jets front, 6.5 PV front and rear, and 73 jets rear.

Average cruise 12.5 A/F

WOT fairly steady at 13.8 A/F

I would like to get the cruise to 14.5 and the WOT at 13.

Can I do this correctly by lowering the primaries and raising the secondaries till I find the right combo, or is it suggested to also modify the primary power valve orifices?

Any suggestions/advice appreciated. Not sure if I am ready to dive into modifying the power valve flow rates just yet and I'm guessing those of you who have done that have really dropped the primary jet sizes for cruise mpg??

Thank you.

[Dwight]

I'm interested in the experts answers

[elmariachi]

I am not an expert, but I have been working feverishly with my Innovate system on a Demon and Holley. In my opinion, with it installed in only ONE of EIGHT exhaust runners you cannot fine tune to the subtle levels you are seeking. For all you know, you are ALREADY at 13.8 on the adjacent cylinder. Swapping out power valves is easy, just do it and see what happens to the numbers. But in my opinion, the Innovate system is not suitable for making decisions about super sensitive tuning elements like power valve orifices and emulsion tube sizes.

[vector1]

your cruise is probably running on the transition/idle circuit and would be changed by reducing the ifr circuits in your case which would require drilling and installing bleeds if not already installed or purchasing bleeds and redrilling smaller. according to a holley book your idle screws should be out about one turn and the ifr's should be about right. if you're still lean probably enlarge the transition slot, but a little goes a long ways so be careful.

raising and lowering the float level can have a small effect but it is not desireable according to the holley book.

you can enlarge the pvcr to increase your wot a/f ratio but you are pretty close as it is, you will need a set of the small bits which you can purchase at the hardware store and enlarge all pvcr one bit size at a time, these are pretty sensitive also.

if the a/f ratio is pretty steady i would not change the 6.5.

make sure your spark advance is where you want it before you start playing with the a/f ratio as this will have an effect on it also.

the holley carb book is pretty helpful in understanding how they work, and this is what little i understand, somebody else might have more insight and be more correct.

using one cylinder to tune is kind of risky as they all will be slightly different i would bet with off the shelf intake and parts.

[Hotfingrs]

First, why are you running PV's on the front and rear?? If you truely have 2 PV's then your jetting should be square, but you should never run 2 on the street.

Second, I only run 2.5 PV, no matter what the vacuum is. You'll have the extra fuel under WOT throttle, but won't be kicking in and out under normal driving, when you're close to the rated vacuum number.

What rpm's are you at, at cruise mph?? Up until 2800-3000 rpm you're on the idle circuit, so your cruise A/F can be controlled by the idle screws, and air bleeds.

You should buy the end of exhaust adaptor for the LM, then you can read 4 cylinders at a time. You'll have to fabricate and extension out of brake line, so the probe is getting a reading at least a foot into the exhaust. It's been my experience that there is to much reversion to get an accurate reading at the end of the exhaust.

It's much easier for me to just tune the carb, then try to write things down....

[randlej]

Thank you. Couple of replies and questions:

1) I am using the Innovate exhaust clamp so the probe is about 8" into the sidepipe. I was planning to have the bung welded into the collector just before the baffle area to see if it varied at all. Right now, readings are very close to the dyno A/F readings I had done recently and the probe was well into the sidepipes then.

2) The carb is a stock holley so it came with the PV in the secondaries. Forgive my ignorance, but I thought blocking the secondary PV was more for the drag strip usage. Also, it has the stock bleed orifices so to change those would require drilling/adapting.

3) My cruise figures were based on about 2,300 to 2,800 light cruise. I'll fiddle with the mixture screws and see if I can lean it out some.

Questions:
1) What is the ideal advance curve on the spark advance? I have it all in at about 2,800. 16 initial, 35 total, and a vacuum canister set to bring idle timing to about 20 and it at full vacuum adds around 8 degrees. Car seems to like it but I'm sure it could be tweaked some.

2) Probe extender w/ brake line. Are you talking about just extending the venturi effect tube in further? or somehow getting the 02 sensor itself in further?

3) Doesn't appear in the replies that I should be changing the jets but more with PV orifice changes. Yes?

[scootter]

I tried to use the exhaust adapter with the my LM1 and the readings were crazy. Worked great with the wifes Escalade, but no way with a high performance engine. I welded bungs just after the collectors on both sides. I have a Demo 850 that came with a tuning guide without using an AF meter. Amazing how close you can get them with out a meter.

Scott

[Hotfingrs]

Easy answer first, yes just extend the venturi tube. Drill and tap the end for a brake line fitting, then about another foot onto the end of the original clamp. The reason for the crazy readings was the reversion effect at the end of the exhuast, that's why you have to be in farther.

I think you're confusing the primaries and the secondaries. The primaries are the front 2 barrels and should have a PV. The secondaries are the back 2, and should not have a PV. A PV will enrich the circuit by approx 6-8 jet sizes, that why the jets are larger in the back then the front.

With your cruise figures you should be on the idle circuit at cruise, or just barely into the primaries at 2800. Do you have 4 corner idle adjustment, and can you hook up a vacuum line?? If you doand can. start with each idle screw out the same amount, then adjust each screw until you get the highest vacuum reading. They should all be pretty close to the same amount in or out. After you have the highest reading on all four, add 1/8th to a 1/4 turn more, you'll find it's to lean if you leave it there. Also drive down the street at your cruise speed and see what you have for a vacuum reading and how close you are to the 6.5 inches.

There doesn't seem to be a problem with your advance curve, Should start coming in around 1000 rpms and be all in by 2800-3000 rpm

[elmariachi]

No matter what, do NOT start mixing timing adjustments and carb adjustments. Ideally, you would have your timing relatively in line before trying to tweak the carb. Assuming your timing is generally within range (your numbers suggest they are) then I'd leave it alone for now. Idle mixture screws are only for adjusting A/F at idle, not down the road. As Jack said, set your idle first with the vacuum gauge and then leave it alone. Then make sure your front power valve value is a couple points lower than your idle vacuum and go drive it. Just to clarify the prior posts, you shouldn't be running a rear power valve and as such, your rear jets will be larger in size versus the front jets.

Seat of the pants is always the first measure, then the Innovate. Once the car idles, transitions and accelerates smoothly, THEN you can always go back and see what effect +/- 2* timing might have. Lastly, spark plugs will tell you if you are running lean or rich and you can use that info versus your Innovate system to fine tune your settings.

I see you are in Lakeway. I work at 2222/620 and am available in the early evenings during the week, let me know if you want an extra set of eyes.

Jim

[randlej]

I do have a vacuum guage and used it in the initial setup. Idle screws are set to highest vacuum and have toyed 1/4 turn +- from there. Idle mixture screws avail only on my primaries.

Cruise vacuum is 15-18

I may be wrong about my secondary PV. I've never opened the bowl to check. I just assumed it had them front and back.

Mariachi, thought you were in N. Houston from your signature line. Do you ever attend the Austin club meetings? Perhaps we've met. In any event, it would be good to meet you.

[elmariachi]

I work in Austin but the rest of my world and my Cobra is still in Cypress for the time being. I believe we may have met in San Marcos. I know Ken, Gregg, Dee and a few of the locals through TCC but have not yet attended any meetings. I am more into driving than congregating.

I did mean to ask, other than tweaking the AF values, how does the car run? Are there other performance issues you are trying to resolve?


Jim

[randlej]

Car runs good. Just being anal

Mariachi, I'll PM you for contact info.

Randy