Read my plugs? This is too lean, right?

[14Accent]

I'm doing my best to learn the world of carburators, so bear with me. These plugs came out of our newly acquired 1979 Cadillac Deville with a freshly rebuilt, bone stock 425 that's fed by an Edelbrock 1406 carb.

From what I've read, the 1406 is a 600 CFM "economy" minded carb, which should be just fine sitting atop a low-comp smog-era 425. While I admit the car runs good, it has a light surge at steady cruise that I'm working on diagnosing.

My process so far:

Verified spark timing per the service manual: 23° BTDC @ 1400 RPM with the mechanical advance vacuum line removed and plugged. I'm sitting just above, maybe 24-25°. The cap, rotor, plugs and wires have less than 3k miles on them, and I verified that they're in good shape. I did find the mechanical advance weights to be very sticky, but a little brake clean and a light spray of WD-40 freed them right up. The current plugs are Champion 3031's, but I've got a set of AC Delco 41-819 double plat's on the way (more my own preference, Champions don't belong in a GM). They're gapped at the correct .060".

I replaced the upper and lower carb gaskets with the kit from Edelbrock, the ones on the car looked terrible and didn't match the ports on the adapter or intake. This helped a decent amount, I'm pretty sure there was a vacuum leak between the adapter and the manifold.

I pulled the metering rods and springs out of the primary circuit and found that someone has installed the "silver" springs which are the most stiff at 8" of mercury. The metering rods are the stock 7547's. I didn't remember to peek down at the jets, but I have to assume they're original spec as well. I have the Edelbrock carb calibration kit for the 1406 on the way, so I'll soon have the ability to play around with these parts.

I set the idle circuits with a tach and a vacuum gauge the best I could. I've got a solid 20" of vacuum at a rock steady 650 RPM in gear, which ended up being ~2.5 turns out on each idle screw.

Here's my initial thought: after reading through the manual and looking at the calibration chart, I'm thinking of going with 7342 metering rods and dropping down one size on the metering springs. This bumps the fuel up 1.5 stages rich on both the cruise and power circuits.

Given the condition of the plugs currently, do the bigger brains in the room agree with this assessment? I'm completely open to whatever tips and tricks you all can share with me.

 

[joegreen]

The plugs look really nice to me but im no tuner.

 

[dbias]

I don’t think it looks lean at all. You want a coffee color, which those have, they are not white/ashy and no build up unless the lighting of the picture is throwing things off.

 

[JWC86]

I can't help with tuning your automotive carbs but I will agree with the others that those plugs don't look lean to me at all.

 

[bunnspecial]

I think most people wish they could get their plugs to look that good!

If I had to split hairs, it actually looks a tiny bit rich to me based on the base of the threads, but I'd not chase it. It also looks to me like you could stand to advance your timing a tiny bit(and I'd do that before messing with the mixture). Ideally you want the color transition on the ground to be right around the bend-retarding the timing moves it closer to the tip and advancing moves it closer to the base. Yours is about 2/3 of the way down the tip, or at least that's how the photos appear, so I'd be inclined to give it 1-2º more and see how that affects things.

 

[SC Maintenance]

They look pretty good to me. Light tan is correct. How many miles on these plugs approximately

 

[slo town]

I think most people wish they could get their plugs to look that good!

If I had to split hairs, it actually looks a tiny bit rich to me based on the base of the threads, but I'd not chase it. It also looks to me like you could stand to advance your timing a tiny bit(and I'd do that before messing with the mixture). Ideally you want the color transition on the ground to be right around the bend-retarding the timing moves it closer to the tip and advancing moves it closer to the base. Yours is about 2/3 of the way down the tip, or at least that's how the photos appear, so I'd be inclined to give it 1-2º more and see how that affects things.

I'm inclined to agree, but they look good enough it wouldn't be worth the trouble to change anything.

Scott

 

[ripcord]

They look great to me. If it's not pinging, surging, or hesitating I would probably leave it alone. Sounds like someone already was tuning on it.

 

[14Accent]

They look pretty good to me. Light tan is correct. How many miles on these plugs approximately

~3k miles

They look great to me. If it's not pinging, surging, or hesitating I would probably leave it alone. Sounds like someone already was tuning on it.

I have no pinging that I've been able to detect, and that's on 87 octane E0. There's an EVER so slight throttle tip-in hesitation when it's not fully warmed up, but moving the accelerator pump rod from the middle hole to the furthest in may cure that. I'll continue to experiment.

My main complaint is a very slight surge at very light cruise: think a slight oscillation feeling at 55-60 MPH on flat ground. Giving it just a squeeze more or coming full off the throttle gets rid of the surge.

I appreciate the tips, everyone. I'm going to wait for the calibration kit to come in and play with things a bit, at least now I have the baseline of where things are so I can revert should I make things worse.

 

[bunnspecial]

I'm inclined to agree, but they look good enough it wouldn't be worth the trouble to change anything.

Scott

No argument from me on that.

A lot of times with carbs you can get "pretty darn good" or drive yourself crazy getting "perfect." Racers who need every bit of horsepower the car is capable of go for "perfect" but are probably also using dynos, tailpipe sniffers, and other tools to get "perfect."

For a car just to drive, I tend to prefer tuning a bit on the rich side anyway-not rich enough that you're dumping piles of gas out the tailpipe or loading up your plugs so much it stalls when idling-but a shade on the rich side isn't usually terrible. Slightly rich, IME, tends to give a bit better driveability plus going lean can burn exhaust valves and cause other issues. You might lose 1-2mpg tuning that way, but really, if you're driving a '79 Deville around, fuel economy probably isn't at the top of your worries.

 

[14Accent]

I think most people wish they could get their plugs to look that good!

If I had to split hairs, it actually looks a tiny bit rich to me based on the base of the threads, but I'd not chase it. It also looks to me like you could stand to advance your timing a tiny bit(and I'd do that before messing with the mixture). Ideally you want the color transition on the ground to be right around the bend-retarding the timing moves it closer to the tip and advancing moves it closer to the base. Yours is about 2/3 of the way down the tip, or at least that's how the photos appear, so I'd be inclined to give it 1-2º more and see how that affects things.

Do you think it would take another couple degrees? I'm seeing about 24-25° right now, I don't want to push things on 87 octane.

I forgot to mention, economy wise I've got no complaints. It's pulling down 14.5-15 MPG avg. over the last couple tanks.

 

[bunnspecial]

My main complaint is a very slight surge at very light cruise: think a slight oscillation feeling at 55-60 MPH on flat ground. Giving it just a squeeze more or coming full off the throttle gets rid of the surge.

Have you verified operation of the vac advance? Manifold vacuum should be fairly high under those conditions, and if vac is operating correctly it might give you the timing the plugs seem to point to the car needing and cure that issue.

If you have the specs for the vac can, it's easy enough to check it with a Mityvac and dialback light. I don't know what the convention is on GM/Delco distributors, but on the ones I've worked with the vac can specifies timing in camshaft degrees, which are half what you'll see at the crankshaft-i.e. a vac can that's specced to start advancing at 5"Hg, be all in by 15"Hg, and give 5º of advance will actually give 10º by the crank. I don't know if yours will be specced that way or not, but just keep it in mind.

 

[ripcord]

You've probably seen it but Edelbrock has an excellent tuning manual.

https://www.edelbrock.com/media/wysiwyg/documents/carb-owners-manual.pdf

According to the chart on page 13, to go 4% richer than stock metering at cruise, you would use the stock primary jet of .098 and and change the rod to #1456 (.073" x .047")

 

[CarlB]

If you just want a passing grade those plugs look fine.

If you would like to ace the class you could buy a simple wideband o2 sensor and clamp to exhaust tip to verify the AFR at a few operating conditions. Beats the guess work from plug reading, since plug reading is more about averages.

 

[ripcord]

Just keep in mind that a carburetor is a device designed to give the wrong AF ratio in all conditions. All carb tuning is a matter of getting it "close enough."

 

[oil pan 4]

That's what they should look like. If the ceramic is glazed then its lean under heavy load which is bad.
Plus lean isn't automatically bad. I installed a wide ban O2 sensor in my old suburban and tuned it to run lean when cruising down the road, but when the secondaries opened up they were fuel dumps, no lean burn when the throttle was opened up more than half way.
The only way to get a perfect tune is a wide ban O2.
After installing a wide ban O2 I immediately realized most of what I had been told about carb tuning was completely wrong.

 

[14Accent]

Have you verified operation of the vac advance? Manifold vacuum should be fairly high under those conditions, and if vac is operating correctly it might give you the timing the plugs seem to point to the car needing and cure that issue.

If you have the specs for the vac can, it's easy enough to check it with a Mityvac and dialback light. I don't know what the convention is on GM/Delco distributors, but on the ones I've worked with the vac can specifies timing in camshaft degrees, which are half what you'll see at the crankshaft-i.e. a vac can that's specced to start advancing at 5"Hg, be all in by 15"Hg, and give 5º of advance will actually give 10º by the crank. I don't know if yours will be specced that way or not, but just keep it in mind.

Vacuum can is working as designed as far as I can tell. I haven't thrown my mighty-vac on it but just using my timing light I can watch the advance change with RPM.

I am running ported vacuum for the advance, maybe switching to manifold vacuum will help? At a steady cruise it runs right about 15" of vacuum, give or take. Idle is 20", and both readings are rock solid.

 

[14Accent]

You've probably seen it but Edelbrock has an excellent tuning manual.

https://www.edelbrock.com/media/wysiwyg/documents/carb-owners-manual.pdf

According to the chart on page 13, to go 4% richer than stock metering at cruise, you would use the stock primary jet of .098 and and change the rod to #1456 (.073" x .047")

That's the manual I've been going off of. I'm thinking of using the #1456 rod but leaving the .098 jet in place and trying that setup out with the "silver" metering rod springs that are currently installed. Someone has definitely been inside this carb and tweaked it, however I've already found and solved a few workmanship issues with the car so I think I can get things even closer.

 

[oil pan 4]

Vacuum can is working as designed as far as I can tell. I haven't thrown my mighty-vac on it but just using my timing light I can watch the advance change with RPM.

I am running ported vacuum for the advance, maybe switching to manifold vacuum will help? At a steady cruise it runs right about 15" of vacuum, give or take. Idle is 20", and both readings are rock solid.

No do not use the "ported vacuum". The ported vacuum elimins or reduces the vacuum at idle, for lower NOx emissions.
Just try it ported vs manifold vacuum at idle. You will likely find it runs much better with manifold vacuum. With full manifold vacuum it engages the vacuum advance at idle.

 

[14Accent]

No do not use the "ported vacuum". The ported vacuum elimins or reduces the vacuum at idle, for lower NOx emissions.
Just try it ported vs manifold vacuum at idle. You will likely find it runs much better with manifold vacuum. With full manifold vacuum it engages the vacuum advance at idle.

Understood, I knew ported vacuum was for emissions but the car still has its EGR valve installed and connected so I wasn't sure if manifold vacuum would give too much advance in combination with the EGR.