GASGZLR

Here is the wideband A/F ratio from my dyno pull. The higher green line is with my full exhaust and the others are with a 3 inch cutout after the Y-pipe. It dips down at about 3500 rpm then goes back up. I think this is where the secondaries are coming in. I am going to jet them 1 stage up ( lean) to try to get it closer to a 13:1 ratio.

This is on an Edelbrock 600 cfm carb. 1405. Primaries are 8% leaned out ( #24 on chart) and the secondaries are stock .095 jets. I am thinking about going to .092 jets.

Can someone help?

GASGZLR

No one? Maybe it is not possible with an edelbrock carb. I need a holley bad.

Damon

Yeah, it leans out very gradually in the upper RPMs. Not like you've got a moon rocket or a submarine on the tail end- that would be VERY bad. Yours is pretty flat.

I would guess, and I mean this is an "internet guess", based on nothing but experience and instinct, that you are starting to outrun the carb in the upper RPMs, build a smidge of manifold vacuum and just ever so slightly lean the mix up top. Nothing to get excited about, but sometimes you'll see it lean out a bit as you start to build just a smidge of manifold vacuum against a slightly too small carb.

I'm more concerned about the lean spot from about 2500-3000. I'll be willing to bet that's from leaning the primaries. THink about it- when you first smack the throttle open at low RPMs ONLY the primaries are providing the fuel and air (in a vacuum secondary carb like your Eddy). As the RPMs rise, the secondaries are opening and providing more and more of the A/F mix. Usually, they're pretty far open by about 3500-4000 RPMs, depending on how tight or loose you have them adjusted and the overall A/F ratio start to stabilize, as yours does.

I would suggest, as something to try for your next dyno testing session, to put the primary jets/rods back to stock and try working on the secondary side. Agreed, it should probably be leaner than it is, but I'm concerned you've spiked it a bit lean in the lower RPMs with your leaner primary calibration.

Latly- about open headers vs. muffled exhaust. Don't try to compare them too much. You have to decide which way you want it to work best and tune it that way. HOWEVER, consider that with open exhaust you're moving more air and getting a stronger intake pulse, you will typically open the vacuum secondaries EARLIER in the RPM range with open exhaust. And vice-versa.

Your A/F ratio chart tends to show this to be the case, especially if you assume what I said above to be true. How so? Well, with muffled exhaust your secondaries will open later. You'll be runnign mostly on the primaries until higher in the RPM range through the mufflers. Remember how I said that your primary calibration may be too lean, as indicated by the obvious low RPM leanness? Well, guess what? You carry that leanness higher into the RPM range with the mufflers corked up. You're running on the primaries more in the lower RPMs becuase the secondaries haven't opened as quickly with the mufflers corked up, if the primaries are too lean, you carry the leanness higher in the RPM. That is until the secondaries open far enough to once again offset it.

Damon

I'm going to try to anticipate your next concern. Please forgive if I get it wrong. You'll say "but my mileage sucks when I put the primaries back to stock, it's obviously richer than it needs to be driving around town."

Fair enough. Edlebrock offers a wide selection of primary RODS to help with this. You can get them with different tip diameters. You could choose a primary rod that has a leaner part throttle metering diameter but the same WOT power tip diameter as stock and use stock jetting. OR, you could use a rod with the same part throttle metering diameter as stock but a richer WOT metering tip and keep using your leaner jets.

For instance, your stock carb probably came with primary jets of about .098 and a .070 X .047 primary metering rod. The first number on the metering rod is the part that meters fuel at cruise/light throttle conditions. The second number is the part that meters fuel at WOT. Remember, bigger is LEANER on the rods (just the opposite of jets).

You could use stock .098 jet but with a .075 X .047 rod and have the same WOT A/F ratio on the primaries as stock, but a leaner part throttle cruise A/F ratio.

OR, you could keep using your leaner primary jets (let's say .092) but use a .070 X .037 primary rod. That would keep your leaner part throttle calibration, but fatten up the WOT mixture on the primaries.

There's a lot of ways to skin the cat with an Eddy. As long as you think about what you're doing, they are very flexible in their tuning range. ANY cvarb can be tuned to work well in most cases. It jsut takes some research, thought and a lot of trial and error.

GASGZLR

You have given me a lot to think about. Here is what i am thinking I will do. Lean out the secondaries, and fatten up the power mode of the primaries while keeping the same cruise a/f ratio.

GASGZLR

No that I think about it, I might try to just get a Holley.

GASGZLR

I did this last night and it seems to have worked great, thanks guys!

front barrels running .098 jet with 73x 52 rod ( now it is a 73x47)
back barrels were running a .095 jet and now they are .092's

Damon

No problem, glad to help out.

If you want to know how I tune carbs myself, well.... you just took a walk through my brain and thought processes. There's not much more to it than what I posted about. Just raw information that turns into an idea that then becomes a theory about why the results came out the way they did. Once you have a theory that seems to fit the facts, you test it by making an hypothesis about whether a certain change would give you an expected (hopefully better) change in results. Then you test the hypothesis by makign the change and see if what you expected to happen actually happens. If so, you're on the right path. If not..... back to building a better theory.

Next thing I'd try is to put in some 73 x 42 primary rods and jet the secondaries even leaner- maybe 2 jet sizes smaller. See if it keep getting better or if you went too far.

If it gets even better, then try a set of 73 x 37 primary rods and jet the secondaries another 2 sizes leaner and see how that does.

A roughly-right rule of thumb with Eddy carbs is that a change of 5 sizes on the rod = a change of ~2 jets sizes.

You're on your way to being the "Edlbrock Carb guru." Having access to wideband A/F ratio results is a big BIG help. Having real data is better than "I did this and I think it feels better."