Carb tuning with air fuel ratio gauge

[Paul G]

Carb tuning by plug reading has never worked for me. The plugs always looked the same. While researching how to use the A/F ratio gauge, I have read  that today's oxygenated fuels along with the additives they use, that plugs dont change color like they did with straight gas with lead additive.

What puzzles me is how the jetting is laying out so far. I am running a Holley Street Avenger 670 on the 360. Ideal is 14.7:1 for gasoline. Ignition timing is 30° base, 34° total all in by 2000 rpm. If timing is a factor?

Column 1 is where I started with the A/F gauge.

In column 1, you can see low speed is a little rich, light throttle is way lean, full throttle is rich.  

In column 2, I went back to the factory jetting for the 670. The numbers changed. It leaned out across the board.

In column 3, I leaned the secondary side only, left the primary alone. All the numbers changed again. Light throttle leaned out, full throttle leaned out slightly. It looks like the primary wants to be richer yet, secondary leaner. The primarys and secondaries look they will end up being very close in jetting? If not the same, or larger primaries?    

After the jet change in column 3 the cars pulls noticeably harder at full throttle.


                                   Column 1                   Column 2                  Column 3
Jetting                  67 p 70 s                      65 p 68 s                      65p 67 s
idle @ 18" vac               12                                12                       12
40 mph                  14                                14.5 to 15                12.9
60 mph                  13.8 to 14                     15.9 to 16                15.5 to 16
light throttle               16                                14.5 to 15                15 to 16
full throttle                  11                                11.5                       11.8 to 11.9

[1974dodgecharger]

I like your No. 1 column  as I always learned a little rich is better than being a little lean.  I installed a A/F also on my driver side collector area and it has taught me a lot of things that I would have never thought of even with reading plugs I was never good at.  I learned my initial shot was leaning out my car badly as in it hit past 17 ratio.  this always happened even on highway speeds let off the gas and stab the gas I get lean for a second.  Played with the pump cams and realized I needed the blue cam and probably bigger squarters, but its gotten better now.


As for crusing I hit 12 to 13 ratio area I may see 11 on occasion.  So im pretty fat.....I blocked off my sec. and run a 82 jets and 72 jets in fronts I was at 74 in front, but I was really fat. 

I enjoy the A/F setup better than plugs that's for sure......

[Paul G]

Does your gauge reading move around a lot? I was expecting to read a number, but the best I can do is guestimate a range because the number is constantly changing.

[XH29N0G]

I have also been working with an A/F and find it helpful, but am much more of a novice that most on here.  What I have read is the below, and I will put it out there because I am sure someone who knows will correct it if I am mistaken.  (I also hooked my A/F to my computer.  I am using one of the innovate models that has an output that I was able to adapt that to a USB with a RS232 to USB adapter I bought at best buy. This is a big help and I recommend it if possible.  

As far as setting up the carb, I am trying to do that now and this is how I am going about it.  I tuned the idle as one normally would (for highest vacuum) - for me this is about 12.9.  The idle circuit should work up to about 2500 RPM at steady throttle, and all you want is it to be stable and I think it is fine if it is around 14.7 give or take.  

The high speed circuit works above about 3000 RPM and I think the way to set it is to cruise at a steady RPM 3500, 4500, etc.. and check the A/F.  I think one wants A/F for this around 13-14 (but do not know how the power valve factors in because it will add gas).

I then think one is supposed to check AF under full acceleration and aim for A/F of 12.5 to 13.2 because that is supposed to be the best.

Finally, I think one checks the transition to throttle by accelerating and looking for spikes (lean or rich) as the gas is pushed and adjusting the accelerator pump cam and shooters to reduce those.  

I think there are many other things that can be done to fine tune, but my guess is this is the way to proceed.  At least, that is what I am thinking I will try.

[Paul G]

XH29N0G, I think you are on the right track. I have read 14.7 is the ideal ratio. Achieving that with a carb is very difficult. I started by returning the Street Avenger 670 to stock jetting, 65 pri, 68 sec. I found that the ratio was off with stock jetting. It was rich, opposite of what I read about the SA carbs. People say they are set up lean. Not what I am seeing.

Rich idle, the best idle quality was @ 12 to 1 ratio, adjusting for highest lean vacuum. For me it is at 16" vacuum. Stinky exhaust smell, so I adjusted it a little leaner, 12.3, engine doesnt idle as smooth but stink is less.

Stock jetting 65 pri, 68 sec
Part throttle cruise under 2000 rpm was 14.5 to 15. Right on the money per say.
Higher speed and rpm richened it to 16. Still okay from what I read on some sources. Going that lean at extremely light loads can be okay, or so I read?
Full throttle went rich at this jetting, 11.5.

Still, running at 16 to 1 or leaner bothers me. Even at light loads?

After some trial and error I have the carb squared off right now, 66 jets front and back.
Light throttle cruising is around 14.5 to 15 pretty good.
Full throttle just a tad leaner at 11.9, still rich. I am going to try one size smaller jets in the secondary, 65's, and leave the primary at 66. Hopefully that gets the full throttle ratio to about 12.5 to 1. perfect.

Going with smaller secondary jetting compared to the primary side is puzzling me. Not what you normally see?

On the Mopower Cruise to Vegas last weekend I was getting from 13 to 16 MPG depending on speed. Best MPG I have ever gotten with this car.
   

[Paul G]

In my understanding, the power valve will open at whatever vacuum rating it is calibrated for. A 65 PV, will open at 6.5" vacuum. As the throttle opens, under hard acceleration, and manifold vacuum drops to below 6.5", the pv opens allowing lots of fuel to richen the mixture with all the air that is entering the engine. If I understand it correctly.