AFR

[Birddog1970]

Hey guys,

I've been trying to dial in my Carb up in in Denver where there is less air than trees. I got a hold of an Air/Fuel tester so I can see the numbers while driving the car. Below is what we came up with. I'm running a mild street build in my 70 396 CTX motor (290 heads, stock intake, stock exhaust manifolds, with some minor head work and slightly bigger than stock cam, etc) with a restored 7040202 carb that had the air bleeds opened up based on cliff #2 recommendations. The air/fuel sensor was mounted at the tail-pipe, not cut into the exhaust. Lastly, it was a cool day today, around high 40's and sunny.

First series of tests with 43B Rods, 71 Jets, and CH secondary rods during driving condors on a flat road

RPM (steady) AFR (Avg)
IDLE 14.7
2000 14.8
2500 14.8
3000 14.7
3500 14.7
4000 14.3
4500 14.0

At WOT holding through 4500 RPM she would dip down for a second around 12.5 then increase back to mid 14's by the time I reached 4500 RPMs

Second series of tests we ran 43B Rods, 73 Jets, CH secondary rods:

RPM (steady) AFR (Avg)
IDLE 14.7
2000 13.9
2500 13.9
3000 13.7
3500 13.7
4000 13.6
4500 13.4

Again at WOT from 2500-3000 RPM would start low 12 to mid 13's and 3500-4500 RPMs it would creep from 13 to low 14's

We ran out of time (and additional jets/rods) but it seems the Jet change from 71 to 73 helped, however,I believe my secondary rods are too large since the car would go lean  as we increased RPMs through the range at WOT.

Looking at those numbers does anyone have any suggestions on the primary jet/rod combo and secondary Rods? What should my AFR goal be while holding steady at 2000, 3000, 4000, etc RPMs? And what is the target AFR at WOT ... I heard in high 12's. When I get into higher RPMs (3,500+) at WOT I feel a surge of initial power when I hit it but then it just seems to go "flat", not nose over just really doesn't feel as if it's pulling harder


Any help is appreciated for my next step.

Thanks,

Dave

[omaha]

I think the second set of numbers are probably closer than you think.The reason is if the fuel has alchohol in it, this will change the stoichemertic to something a bit richer. Of course, without the alky, normal stoich is right at 14.7. So maybe you are just a tad rich  but ok with the bigger jets. With the rods, I would find some old rods, similar to the ones you have and just modify them a bit. use a small file and a drill motor andtrim just a little bit. use a micrometer to get them the same. Another option would be to "bend" the secondary rod hanger slightly so it rases the rods just a bit. A good set of calipers for measuring works good in this situation. Different hangers are also an option and so is drilling the hangers for the rod location a little higher or a combination of all 3 modifications. Just keep plugging away at it slowly. the good thing is that the secondary rods are accessible without taking the carb apart (thank God). Hope this helps! also, make sure that your float is set right usually 1/4 inch and that you have a good needle and seat that flows enough and that your fuel pressure is not dropping of at the upper rpm's. It's ok to be rich on the secondary side, you just don't want to go excessive. Good luck!

[Shark Racer]

I think the second set of numbers are probably closer than you think.The reason is if the fuel has alchohol in it, this will change the stoichemertic to something a bit richer. Of course, without the alky, normal stoich is right at 14.7.

Be careful, when working w/ a wideband typically the AFR number is a scaled value of lambda and lambda does not change.

1 lambda is stoich, regardless of fuel content. So when it displays 14.7, it's actually displaying stoich. It doesn't matter if it's E-0, E-5, E-10, or E-85. The lambda sensor is unaware of the ethanol content in the fuel and just displays whether or not the combustion process hit equilibrium (stoich will have the combustion process at equilibrium).

When I got started tuning (EFI) I tricked myself this same way and assumed the numbers coming off my WB controller (a FAST model) were AFR when in fact they're just a scaled representation of lambda.

In addition to that, OP, it sounds like you're just cruising down the road and checking AFR? Or are you doing this on a dyno at WOT? With steady RPMs in the lower RPM range I'd wanna see numbers closer to your original numbers. When you go deeper into the throttle and increase the load the rods will rise out of the jets and automatically give you a larger #. In other words, you tune cruise with RODs and WOT with jets on a Q-Jet. That said, modifying jet size impacts both cruise and WOT, so you *start* with tuning jets and get the desired AFR under load and then look at the rods to fine-tune your cruise mixture.

I would not be too interested in playing with the primary side AFR over about 3000 RPM because, as Omaha said, you should let the secondaries handle that.

For primary side tuning, the secondaries should be disabled or you need to be careful not to engage them. Hill passing is a good way to test, simply find a tall hill and give the car about 50% throttle (the secondaries do not engage mechanically until roughly 2/3 travel, verify on your carburetor), and do your testing that way. Lock down the jet size that you want and continue with rod tuning from there.

Hope that helps!

[Cliff Ruggles]

Carb number 7040202 is a larger MAB carburetor and if anything would need smaller bleeds driven into it to tune with the parts you are using.  They require pretty larger jets and small metering rods due to having so much emulsion air to the main system.

I would try to remove the factory APT screw in the baseplate and get one of our external APT screws, and custom jets/metering rods so you can tune the carb without taking it apart and changing parts.

It takes some care to effectively remove the APT screw in those baseplates, but if you take your time, use some heat, penetrating oil, and a good fitting screwdriver, it will come right out.....Cliff