AFR tuning

Norris McCarty

Norris McCarty

Member
I’ve a 40 coupe with a 53 Merc that has a Max1 cam, Navarro heads, 3x2 Navarro intake with 2 real 97 Strombergs setup to run together(much better than running the 3 progressive) and Reds headers. I’m also running one of Charlie’s SBC mechcanical advance distributors. This engine has about 2000 mile and runs good.
I can’t leave well enough alone, so I installed an Innovate AFR gauge in the glovebox.
I’ve just started tuning.
With both carbs set perfectly with a vacuum gauge it idles with an AFR of 12.7.
I understand that a perfect idle should be 14.7 on the gauge, but if I close the mixture screws in enough to lean out the mixture to around 14 on the gauge, it doesn’t idle near as good. Does this mean that I have a problem somewhere? Perhaps a vacuum leak? The car idles beautifully at 680 rpm.
As I get into cruise tuning I’ll report back.
Just curious....I know most would just not worry about it.
 

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I have the same AFR gauge on my engine test stand. I have found that these engines seem to want to idle richer than they run at cruise. In my experience, what you're seeing is normal.
 
I have both AFR meters and EGT bungs in the headers of my 284 cubic inch Merc. Also, I've done a LOT of dyno tuning of various race/performance engines. Don't spend any time worrying about idle AFR readings - if your initial advance is correct and you've set the idle screws to optimize the idle, then you're fine. Also, there is nothing really very "tuneable" in the idle circuit . . . your jets and power valves have nothing to do with it.

Where an AFR meter is more valuable is in taking the car on the road and seeing what the AFRs are at part throttle, mid-throttle and full-throttle . . . as well as when you hit the accelerator hard. You'll probably find that when jetted correctly, that you'll be on a richer side than the "perfect" 14.7 number - especially in a performance engine. While I'd love to care about only mileage, I tend to care more about performance and detonation. I like to see my AFRs under heavy throttle to be in the 13 - 13.5 area . . . as this gives me some safety in the various conditions I'm sure to see.

In my last set of runs, I was actually a bit richer than this under full throttle (around 12 to 1), but the engine ran really well, the plugs looked good, and I'm not really worried about mileage. It is important to consider what are your goals?

I wanted a conservative tune with plenty of safety margin for various environmental conditions - so being around 13 was just fine for me.
 
Thanks guys.
I’m anxious to check where I am at cruise and power in the next day or two.
I have 43 jets in the Strombergs now and hoping they are close to correct.
This is my 3rd flathead car over the 50 plus years of driving and my first hot rod flathead.....I’m absolutely loving it!
My ears are open for any advice you may have.
Thanks for sharing your experience.

Oh yeah, my goal is to have the most efficient running flathead I can possible have.
It performs very well now, but I want to make sure it’s the best it can be.
 
Making sure your timing is correct is the most important initial tuning thing you need to do (before worrying about AFRs).

I usually run about 24 degrees total advance in a naturally aspirated street engine . . . never more or less than 2 degrees off of that. You need an accurate timing pointer (which 32 - 48 engines did not have) - in order to know where your timing is at. Get this sorted out first, then worry about the fuel circuit.
 
I’ve played with the timing for several weeks now. I guess my initial could be a few degrees higher than it should be.
I’ll try setting it at 4 degrees initial and see what happens......I’ll report back on that too.
I’m thinking it’ll also lower my idle vacuum a bit when I do?
I love it how if you change one thing you better get ready to adjust everything else!
Love this stuff....
 
First, the most important tuning should be the "Cruise" AF and timing. Next low speed acceleration and Idle mixture and initial timing are not that important.. Your looking for performance and economy.. On an engine like yours. ignitopn timing in cruise should be 26/28, degs, AF F 14.5/15. A at WOT about 20 degs, and AF 12.7/13.2 AS long as the engine starts the timing and AF are ok. The efficiency of the combustion chamber makes a difference.
 
I have basically identical engine in my ´50 and I also use Innovative AFR meter.
And I have same result , idle mixture needs to be on rich side to run nice and fine.
Like all have said , initial adv. is not that important , but curve and total adv.
Keep tuning.....it´s fun
 
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Idle AFR richer than Stoic are normal especially with a performance camshaft. There is a wide range of AF ratios which seem to run ok in the Flathead engine. Getting to the theoretically perfect 14.7 is not a important goal for many reasons. In fact a "cruise" ratio has been reported in the 16 and 17 figures with proper spark timing and reasonable mixture distribution. Cruise timing is usually around 8 degrees higher than full throttle timing. In fact all my tests show a measurable power improvement by using high RPM retard chips of 4 degrees from 22 degrees after speeds of around 4000 RPM. So, there are several things to consider beyond purely the readings of a meter which may, or may not deliver accurate numbers. The meters can help us decide if we are significantly off base and on which side of the base. But they are not the tell all method for reaching our goals. And, our goals can vary by individual desires.
 
Lots of good info!
Beautiful day here today.....perfect for a drive to the office checking the AFR gauge at cruise for the first time.
I’ll report back this evening
 
One thing I forgot to mention about my air/fuel/spark management system; I installed a Winterburn CDI on the pass. side kickpanel a couple of weeks ago. The main thing I noticed after installation was quicker starts both hot and cold and maybe a bit "crisper" throttle response. I'm running NGK B6Ls gapped at .035. I have used these Winterburn boxes on my hotrod VWs and Porsches through the years. Never had a minutes problem with them.
They are also period correct for my 40 Coupe that's built as an early 60s hotrod.
Winterburn started building these in 1962.
Oh, and if you keep the distributor shaft greased the points will last forever....they are only a trigger with this CDI unless you switch the box to Kettering.
http://capacitordischargeignition.com/

More info on my initial observations regarding the AFR gauge tonight.
 
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Norris McCarty said:
One thing I forgot to mention about my air/fuel/spark management system; I installed a Winterburn CDI on the pass. side kickpanel a couple of weeks ago. The main thing I noticed after installation was quicker starts both hot and cold and maybe a bit "crisper" throttle response. I'm running NGK B6Ls gapped at .035. I have used these Winterburn boxes on my hotrod VWs and Porsches through the years. Never had a minutes problem with them.
They are also period correct for my 40 Coupe that's built as an early 60s hotrod.
Winterburn started building these in 1962.
http://capacitordischargeignition.com/

More info on my initial observations regarding the AFR gauge tonight.
Click to expand...
Interesting, had virtually the same results after installing E3 spark plugs.
Using Pertronix ignition.
 
Ok, here are my initial findings:
Beautiful 68 degree day with no wind. Flat straight highway.
680rpm 12.3AFR
1200rpm 45mph 12.8AFR light throttle 5th gear
1600rpm 55mph 13. AFR light throttle 5th gear
2000rpm 70mph 13.3AFR light throttle 5th gear
2200rpm 75mph 13.5AFR light throttle 5th gear

Full throttle acceleration from 50mph to 65mph 14.3AFR in 5th gear
Full throttle acceleration from 70mph to 80mph 15. AFR in 5th gear
On both full throttle runs the gauge hit the higher number listed immediately then dropped to more rich #s quickly.

Both 97s have 43 jets.
This is all with an initial timing of 7 degrees...27 all in at 2200.

Does all of this suggest I should drop to a couple of jet sizes smaller?
 
I would not lean it out - I'd be very comfortable with those numbers. Keep in mind that your AFRs spiked on full throttle, so you might want to check your accelerator pump settings - is the "push rod" on the outer ball in the lower linkage - the 'W'inter setting? If so, then you're getting the max travel and squirt. Next, you might change the power/enrichment valve to richen it up a little bit (More volume earlier in the cycle). I'd move it one step smaller - then retest. Would be nice to try to reduce the lean spike on the initial acceleration hit.

Important Note: Power Valves use engineer’s number drill sizes, so a smaller valve has a higher number.

Also, you should experiment with your timing - as 27 degrees might not be the magic number. I would move it down to 22, do some more tests and then move it two degrees up to 24, then 26. I run a max of 24 degrees on most of my engines.
 
It looks like it's leaner at WOT than it is when cruising and light load. That tells me you need larger power valves (more flow, not larger number valve) to richen up the wide open driving.

Sal
 
I think the timing is spot on, for cruise, and the cruise jets ate too rich. I;ve never been able to tune the strombergs as easy as the 94's
 
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