A/F Ratios and Jetting a Holley Carb

[70RoadRrunner]

Hello everyone.

I have a 70 RR w/383.

Recently I started data logging my A/F ratio as I drive around the area (in town <50 MPH).

When going through the 3 gears (727 w/RMV) I am seeing readings anywhere from 10-18 AFR. A majority of the time it was 12-12.5.

Suggestions on what I should change (jets) in my carb?

They are now 70 primary / 75 secondary.

 

[68 HEMI GTS]

Ideally cruise should be mid 13 range to 14 for a performance oriented engine. Stockers you car run a bit leaner. WOT is usually ideal somewhere in the 12’s. Some like high 12’s some low 12’s. Depends on the engine combo and the fuel. It’ll usually take more than just jet changes to really get them dialed. This chart is a good reference.

this is my 600 hp 511 cruising down the road with a dominator. It takes lots of work to get them there.

 

[RJRENTON]

What is the brand, model of carb are you using? Since you listed both primary and secondary metering jets, it must be a model Holley 4150....yes? If so, are you using a secondary power valve and its opening point (# on vslve). There are many factors influencing A/F ratios: Cam, spark advance, car weight, final drive ratio, compression ratio, etc. Instantaneous A/F ratios are nice to know but cannot be compensated for while driving, like an Electronic Fuel Injection system. Probably the best determination of OVERALL (all aspects of the operation not just elapsed time) fuel ratios is the spark plug coloration. Others may have differing opinions.
BOB RENTON

 

[70RoadRrunner]

Thank you 68 and RJ.

 

[70RoadRrunner]

Holley Avenger 770 CFM. 6.5 power valve.

245 @ 0.50, .150 Cam

Guessing 3600 lbs with me in the car.

4.10 Rear end

 

[RJRENTON]

I assume that you have read and understand the info presented in the Holley instructions....yes?
Instructions for Part# 0-80770 is very explicit. These instructions also note you MUST install the transmission stud/bracket on the throttle arm to allow the trans to shift correctly....did you do this?
These instructions also show the out of the box parameters for your carb. There is no panacea....the instructions suggest reading the plugs to determine the AVERAGE engine conditions and then YOU must make the determination to richen of lean the fuel mixtures FOR YOUR OPERATING CONDITIONS. Alot of the determination also depends on the manifold being used both intake AND exhaust, as these also factor in equal mixture distribution.
As i mentioned previously, INSTANTANEOUS A/F ratios CAN BE MISLEADING .... they can provide an overall TREND analysis but average conditions are better analysed by reading the plugs. There are many opinions as to these results by brand and heat range of the spark plugs being used. Your best bet now is to just run what you have for a few hundred miles b4 adjusting the fuel mixtures via jet change. Just my opinion of course.
BOB RENTON

 

[70RoadRrunner]

Thank you RJ.

I have read the instructions and been using this carb for at least 8 years without changing any of the factory parts.

The car runs fine, just looking to make sure I get the most safe power I can out of it.

I have adjusted the idle mixture screws using a vacuum gauge for best vacuum and adjusted the idle screw to achieve a good idle while in gear.

I have a manually shifted reverse manual gear body on the 727 transmission so I did not hook up the throttle arm.

 

[RJRENTON]

Thank you RJ.

I have read the instructions and been using this carb for at least 8 years without changing any of the factory parts.

The car runs fine, just looking to make sure I get the most safe power I can out of it.

I have adjusted the idle mixture screws using a vacuum gauge for best vacuum and adjusted the idle screw to achieve a good idle while in gear.

I have a manually shifted reverse manual gear body on the 727 transmission so I did not hook up the throttle arm.

The throttle arm and the associated linkage and transmission connection adjusts the governor pressure and clutch apply pressures. More throttle /governor pressure insures better (firmer or harder) clutch apply pressures and better shifts. I would think even you are manually shifting the trans, under racing conditions you would want maximun clutch apply pressures to reduce/eliminate any slippage. Unless your reverse valve body eliminates any governor control and provides maximum line pressure. In addition, you have probably eliminated the clutch apply accumulator function as well.
BOB RENTON

 

[Nate S]

The throttle arm and the associated linkage and transmission connection adjusts the governor pressure and clutch apply pressures. More throttle /governor pressure insures better (firmer or harder) clutch apply pressures and better shifts. I would think even you are manually shifting the trans, under racing conditions you would want maximun clutch apply pressures to reduce/eliminate any slippage. Unless your reverse valve body eliminates any governor control and provides maximum line pressure. In addition, you have probably eliminated the clutch apply accumulator function as well.
BOB RENTON

The governor pressure is controlled only by driveshaft speed (and the weights). Line pressure is definitely affected by throttle pressure on the kickdown valve / spring. That said, a manual shift automatic is generally re-ported just for this and no longer needs the throttle pressure for appropriate line pressure.

 

[BSB67]

........looking to make sure I get the most safe power I can out of it.

Only one way to do this. Take it to the track with a hand full of tools. Everything else is guessing and speculation. I see more people taking hp out of their motors in their quest for perfect O2 meter numbers.

 

[Jcharger]

I agree with the track comment. A/F gives you a lot of information. Information that can cause you to over think the problem. Read a book or the internet on the different circuits of the carb and what they do. You need to log it to see the full story. If you are just reading, your eye can focus on the high, or low values and miss the average.

I set the Jets for the WOT A/F (from 3500 to 5000) full acceleration and full throttle (aiming for something in the 12.5 to 13.2 range at the top and 11-12 at the bottom). This is a starting point. The best way is to use the track to get the best combination for time. I set the idle to highest vacuum and open up 1/8 turn to make it a little richer and avoid stall. A/F gauge can still give information. If the timing is too low I have had a richer setting. I use the A/F to see where the cruise sits (1800-3000 RPM) because I don't want it too rich. This can be 15 plus or minus. as long as it doesn't buck I am fine. I use the A/F to see how the car tips in to acceleration, but I often see it rise to lean values (16, 17 or even 18 and then drop to low values 10, 11,12) if I really stomp on it. If it recovers quickly, consider myself fine. I have also used it to adjust the accelerator pump and pump cam and discharge nozzles to change a little to change this. If it works and gets you there fast, without some big symptom, you are probably pretty close.

 

[70RoadRrunner]

Thank you for the suggestions.