Rich at WOT, lean at idle and cruise

[modrummer]

Hey everyone,

I have a quadrajet # 17059219 on my 79 Corvette.

Running #72 jets, .041k rods, and CH secondary rods.

Car has Brodix IK200 heads and a Lunati 268 Cam with longtube headers and side pipes.

I'm running an AEM Wideband 02 gauge for tuning.

Currently, at WOT I have about 11.5:1 A/f (too rich)

However at idle and cruise (once warmed up and choke off) I am getting 16:1 or even higher sometimes A/F (too lean).

What should I be doing to even this out? If I change to bigger primary jets to richen the idle and cruise, my WOT will be WAY to rich.

Thanks

[77cruiser]

How far do you have the idle screws out? Do they have any
affect in or out?
When you use a larger than stock cam it lowers the vacuum & usually need some tweaking to the idle circuit.
16-1 at cruise is good if it runs without stumbles or surging.

[Cliff Ruggles]

Right to start with 72 jets are really lean in one of those carburetors, and not surprised it's way off the mark on the primary side. 

We can supply a much better jet/rod combo for the primary side.  Get it dialed in there first, then go after the huge secondaries to tune full throttle.

I'd add here that running 11 something to 1 at WOT isn't really a big deal, better than too lean as EGT's shoot pretty high as does potential for detonation and engine damage.

There really isn't a lot of power gained by playing around with A/F at full throttle anyhow.  I've tuned my carb at the track from "pig" rich to so lean the engine was surging just a tad in high gear and the difference in ET and MPH really isn't all that much, seldom much over a tenth and less than 2mph.......FWIW......Cliff

[modrummer]

Right to start with 72 jets are really lean in one of those carburetors, and not surprised it's way off the mark on the primary side. 

We can supply a much better jet/rod combo for the primary side.  Get it dialed in there first, then go after the huge secondaries to tune full throttle.

I'd add here that running 11 something to 1 at WOT isn't really a big deal, better than too lean as EGT's shoot pretty high as does potential for detonation and engine damage.

There really isn't a lot of power gained by playing around with A/F at full throttle anyhow.  I've tuned my carb at the track from "pig" rich to so lean the engine was surging just a tad in high gear and the difference in ET and MPH really isn't all that much, seldom much over a tenth and less than 2mph.......FWIW......Cliff

Ok so I had some other jets sitting around (74,75. And 78) so  I put the 74 jets in.

Measured my float height and its .300" . Checked my piston height and the screw is 4 turns out from bottom.

Went out and drove it to test. Once fully warmed up, idle A/f is about 16:1/17:1.

Cruise and normal driving in gear is right around 12.5:1_13:1.

WOT is the biggest problem. It bogs HARD under Wot And the A/F gauge pegs to 10:1 until it gets higher in the RPM range and then it flattens out around 11:1.

Thoughts?

[77cruiser]

Do you have any different secondary rods? Idle could use some work too.

[modrummer]

Do you have any different secondary rods? Idle could use some work too.

I dont have any other secondary rods, but I didnt think a secondary rod change could make enough of a difference to fix a 10:1 rich condition.

For the idle being so lean, should I be drilling the mixture screw holes a little bigger ? Or possibly the idle tubes ?

[77cruiser]

May have to do both. Could try the idle mixture holes first.

[modrummer]

May have to do both. Could try the idle mixture holes first.

Ok I'll start with that. What should I do on the WOT rich bog? It didnt start till I upped the jet size to a 74. Should I try jetting back down to 72 and then raise the power piston height and drill out idle mixture screws and hope that's enough to make the idle and cruise rich enough ?

[77cruiser]

What do you have for rods & hanger? Windup on the air valve spring?

[Cliff Ruggles]

The idle system is a BIG player here and you need to start there.  Idle fuel feeds the main system right off idle and thru transition via the transfer slots, so it is part of the primary side calibration.

Never use used jets and metering rods for custom tuning, they will cause lessons in humility.  Always use new parts, if you want accurate results and very, and I mean very fuel jets will pass a pin gauge test and not be worn considerably on the inside from the metering rods being in them for tens of thousands of miles.

Also, K series rods provide very little range of adjustment, we have full tapered rods for that part of the tuning process.

Start with idle tuning, then primary side jet size, then fine tune part throttle A/F with the APT system.

We have the right parts to get you a much better baseline to take a lot of the guesswork out of the equation.  This will include jets, metering rods, power piston spring, idle tubes and correct sizes for the DCR's and bypass air.........Cliff

[modrummer]

What do you have for rods & hanger? Windup on the air valve spring?

Rods are CH rods. I'll measure the hanger height,  but I believe it is stock height, so if anything I would assume it would need to be bent down a bit to lean out the secondary side.

[modrummer]

The idle system is a BIG player here and you need to start there.  Idle fuel feeds the main system right off idle and thru transition via the transfer slots, so it is part of the primary side calibration.

Never use used jets and metering rods for custom tuning, they will cause lessons in humility.  Always use new parts, if you want accurate results and very, and I mean very fuel jets will pass a pin gauge test and not be worn considerably on the inside from the metering rods being in them for tens of thousands of miles.

Also, K series rods provide very little range of adjustment, we have full tapered rods for that part of the tuning process.

Start with idle tuning, then primary side jet size, then fine tune part throttle A/F with the APT system.

We have the right parts to get you a much better baseline to take a lot of the guesswork out of the equation.  This will include jets, metering rods, power piston spring, idle tubes and correct sizes for the DCR's and bypass air.........Cliff

Ok I'll work on getting the idle dialed in first.

Both the 41k rods and the 74 jets are pretty much brand new (bought them last year when I was doing some tuning) so that shouldn't be the issue.

So my cruise is currently a bit rich with the 74 jets, but I think that will be dialed in by lowering the primary piston height a little bit. Hopefully drilling out the idle mix screws (or going to bigger idle tubes) will richen up the idle. So the issue I'm left with is the absurdly rich WOT mix (again, it's like 10:1 or richer since the gauge pegs at 10:1). Is a rod change enough to fix that? It seems like the secondary rods couldn't possibly lean out the mix that much, but then again I am not the expert haha.

[Cliff Ruggles]

Do you have one of our books?

The idle mixture is calibrated by matching idle tube size, DCR's, bypass air, and idle airbleed sizes for the engine combination.

I would NOT open up the holes under the mixture screw very much on that model as it uses fine pitch metric idle mixture screws and they may not seat if the holes are much larger than about .090".

As mentioned, the primary side calibration is lean for one of those carburetors, assuming it is a large MAB model, which it should be.

With he .118" MAB's most of those carburetors had much larger jets in them than what you are using and they were still extremely lean on the emission calibrated engines they showed up on.

Being too rich with small jets, and rich at WOT with the larger CH metering rods may indicate a fundamental issue someplace........Cliff

[modrummer]

Do you have one of our books?

The idle mixture is calibrated by matching idle tube size, DCR's, bypass air, and idle airbleed sizes for the engine combination.

I would NOT open up the holes under the mixture screw very much on that model as it uses fine pitch metric idle mixture screws and they may not seat if the holes are much larger than about .090".

As mentioned, the primary side calibration is lean for one of those carburetors, assuming it is a large MAB model, which it should be.

With he .118" MAB's most of those carburetors had much larger jets in them than what you are using and they were still extremely lean on the emission calibrated engines they showed up on.

Being too rich with small jets, and rich at WOT with the larger CH metering rods may indicate a fundamental issue someplace........Cliff

Cliff, interesting point.

I can measure my MABs and see what they check out as.

I'll order one of your books, been meaning to do that.

I'll try to find time to dig into the carb again. It seems like the main issue is on the secondary side, I'm wondering if when I go to WOT the secondaries aren't opening for some reason... the air door opens easily, and the lower butterfly valves open up when the peddle is all the way down so it's not a linkage issue, but maybe a vacuum issue? If the secondary air door doesn't open that would definitely make it go crazy rich at WOT... I may attach a little go pro camera under the hood and run it with the air cleaner off to see if I can see the door opening.

[77cruiser]

How long does it take for the pulloff to release from fully pulled in to all the way out?