Carb tuning with air fuel ratio gauge

[Paul_G]

I installed a Summit brand, which is an AEM A/F ratio gauge in my 72. Now that it is working I am trying to get the carb dialed in a little better. The readings bounces around a lot, so the numbers I am posting are in a range.

My set up is a 360 engine, stock stroke, iron head, mild head porting, Holley SA 670, RPM Air Gap intake, long tube headers, cam unkown, 2500 stahl, 391 gears. On the dyno it has proven to be a torque motor. 255Hp with 360 TQ at the wheels. Ignition timing is 30° base, 34° total all in by 2000 rpm. If timing is a factor? This engine loves lots of static timing. I had an off idle stumble until I brought the timing up.

What puzzles me is how the jetting is laying out so far. Ideal A/F ratio is 14.7:1 for gasoline right? I have been reading up on how to use the A/F gauge. This is what I should try to achieve; Idle 12.5 to 13.2, Cruiseing 14.7 to 15, Max Power 12.6 to 12.8.


Column 1 is where I started with the A/F gauge. Carb was jetted up from stock. Column 2 is jetting back to stock for the 670. Column 3, leaning the jetting started to improve the ratio.

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

In column 2, Stock jetting, 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

 

[YY1]

Interesting.

Please continue.

 

[Hemi Rebel]

You should have closer to an 8 jet size difference from primary to secondary.

 

[Moparmatt72]

Just wondering with the jet changes are you adjusting the air/ fuel mixture screws? 11.5 is usually where forced induction love to be around. How do your plugs look after a hard run with your current column 3?

 

[Paul_G]

Just wondering with the jet changes are you adjusting the air/ fuel mixture screws? 11.5 is usually where forced induction love to be around. How do your plugs look after a hard run with your current column 3?

Idle mixture runs best about 12.2 for a nice clean idle. Any leaner and the engine idles poorly. I haven't pulled a plug after a run. I will try it and see how they look. I am thinking about swapping the front and rear jets and see how it responds. That would have the larger jets up front. Which seems backwards.

- - - Updated - - -

What effect, if any, does all that base timing have?

 

[Hemi Rebel]

You must have a low compression "smog" motor to be able to get away with 30 degrees base timing. In Arizona to boot. If it works though, it works.

 

[Paul_G]

An update. As for the idle, I leaned it a touch trying to get the rich exhaust stink to stop. Brought the A/F ratio up to about 12.3. Idles a little rougher, but acceptable. It sounds meaner like this, with the dumps open.

I tried changing both the primary side and secondary side to 66 jets. That change richened up the cruise and light throttle to about 14.5 to 15 to 1 ratio. That is better for cruising, but the full throttle was still too rich at 11.6 to 11.9. I am trying to get full throttle to the mid 12's A/F ratio.

The next change was to swap the secondary jets for 65's. Cruise and light throttle A/F ratio change was little if any. Full throttle leaned out to around 12 to 12.4 to 1. I think that is getting close to ideal.

So currently it has 66 jets in the primary and 65's in the secondary. It feels much stronger now than before the jet changes. It pulls harder on the top end, or so it seems. I am going to drive it this way for a while.

The plugs have oil soot on them. I believe valve guide seals are getting tired. Last compression test was pretty good, from 160 to 175. I am going to get a new set of plugs soon and see what they look like after a drive. I will post pics.

On the Mopower Cruise To Vegas last weekend I got the best MPG ever. 16.5 MPG on the way up, stayed under 65 mph, 13ish on the way back, 75 mph. That was with 66 jets front and rear. The car ran strong. I was wanting badly to put it on the track.... But I didn't.

 

[HT413]

This is a great thread, I bought an AEM unit last year and plan to finally install and start tuning this summer. Thanks for sharing your results.

 

[bobtile]

What Hemi Rebel said. 8-10 jet sizes on that carb and don't switch primary and secondary jets.

 

[Paul_G]

What Hemi Rebel said. 8-10 jet sizes on that carb and don't switch primary and secondary jets.

That is puzzling because the gauge doesnt justify that much jetting. It was too rich at WOT with stock jetting.

 

[Hemi Rebel]

That is puzzling because the gauge doesnt justify that much jetting. It was too rich at WOT with stock jetting.

This is what is going on. The power valve is the equivalent of 8 jet sizes. So at wide open throttle the primary jets plus the power valve equal the secondary jets. You will need to play with it a little. I would start over with properly spaced jets and fine tune it from there.

 

[bigmanjbmopar]

I just bought a A/F gauge with o2 sensor going to install it right after a dyno run then tune from there. Hoping to get a good idea of where its at with this new QFT 780. Still need your help Alex

 

[Mike Gaines]

I am going to buy an Air/Fuel Meter but I am sold on the F.A.S.T. Dual sensor meter for $417.00 from Summit. I will be using the meter for jetting readings ONLY at the drag strip where I will be shooting for 12.5-1 readings on both right and left sensors at wide open throttle of course.

 

[pwtjr]

great info, I'm in process of something similar.

 

[Paul_G]

The car is running great right now. A little on the lean side at cruise, but better for fuel economy in a street car. The jetting right now is 65 jets front and rear, Holley calls this "squared off". Stock jetting is 65 front, 68 rear. The car pulls hard and runs great. I have read so much about the SA670 carbs, most are saying that they come set up on the lean side from Holley. I have found that information to be somewhat incorrect. I leaned the secondary side to achieve the desired full throttle A/F ratio I was after.

I initially enriched the carb from stock jetting based on information I was reading online. The car was sluggish, running way to rich. Information on the internet is not always correct. One size does not fit all I guess.

Jetting right now is 65 front and 65 rear

At cruise speeds and very light throttle 14.9 to 15.5 / 1
Full throttle runs 12.6 / 1

btw, running the stock 6.5 power valve

 

[Paul_G]

Something puzzling is going on.

Last night we took the car up the hill (slang in Phoenix for going north to high country) on a road trip with friends. Cruising between 60 and 80 MPH the afr's were in the mid 14 to mid 15 range. Very nice. When we got to the hills it wasn't so nice. When I got on the throttle for longer periods of 10 to 15 seconds, passing slow trucks and just ascending the hills and holding speed, afr's dropped to the very rich area, in the low 11's. The longer the secondary's stayed open the lower (richer) the afr got. I never saw this before because around town on the back roads I could not stay on the throttle that long. Short wot bursts are in the mid 12 afr range on the back roads around town. Last night I could feel the power dropping off as the afr's got richer.

I am puzzled now. The secondaries are leaned out three sizes from stock 68 jets to 65's. This carb should be running very lean at WOT and it is not. I wonder if something on the secondary side of this carb has been hacked to richen it up, PV circuit?

 

[mentor70]

an ignition problem would make you see a rich mixture, as well as a stupid high elevation change.

i grew up in Salt lake city...3600 feet above sea level, all Holleys out of the box needed a 4 size leaner setting out of the box, then tuning off that