johnt365

I have a wideband Air/Fuel gauge and I have spent many weekends trying to optimize a Holley carburetor on my small block. The hardest part was just off idle and light cruise. Lots of time messing with little wires in the idle feed restrictors etc. Well, eventually I stripped the fuel inlet and decided to go back to a Quadrajet.

I had previously rebuilt and run this Qjet but did not have the wideband at the time.

Right off the bat I noticed the Quadrajet was much easier to get a consistent idle reading. It had .071 jets and .044 primary rods and DA secondary. At cruise it was reading 13.2ish and at WOT it was about 11. It seemed rich for a cruiser.

I tried 69 jets and that was cruising around 17.5:1 + , too lean. I went to a .070 jet size and CH secondary rod and think I am there.

Idle in gear = 13.5
light acceleration = 13.7-14.7
Off idle/part throttle cruise = 15.5
WOT = 12.5

I think I am done. I am skittish about the leaner mixture even though I have been trying to get a leaner cruise ratio. It does not surge or ping. At 60 mph in overdrive, the a/f reads in the mid to high 15 range. As I start to climb a hill and apply more throttle, the a/f goes down to a safer 13.5-14.7.

Am I pushing my luck with the 15-16 air fuel numbers at light throttle or can I stop now and call it done?

zwede

There's no risk of engine damage as long as it's only lean at part throttle. Too lean makes it miss and chug. If you don't notice any of that you're fine.

You may find it runs too lean when the weather cools off though.

johnt365

I just got back from a mix of highway / city driving. The numbers are a little different than I originally posted but still no surging or performance missteps that I can feel.

Cruising in 3rd gear 40-50 mph around town =15.5
Cruising in overdrive 70 mph @ 2k rpm on the highway = 16.9-17.2

I got about 18.8 mpg on this little trip. WOT is still 12.5 and the power piston spring seems very responsive to extra load needs.

If I didn't have the A/F gauge, I would not know it was lean. I am wondering if I should change to .043 rods to enrich the hwy cruising a bit or should I disconnect the gauge?

Shark Racer

What are your RPMs in 3rd @ 40 mph?

I'd try to get those numbers at 70 down to the 15s. I'm surprised you don't feel any surging. Is it an automatic? (That may be hiding some of the roughness)

My 78 was running a bit lean but I didn't notice with the auto. When I switched to the manual the car felt like complete garbage. (not saying that's the case for you, just iterating my experience). A torque converter can hide a decent amount of stumble.

v2racing

You will often get better mileage at cruise set at stoichiometric than you will with it leaner than that. Aim for around 14.68 on straight gas and 14.3 for ethanol blended. The car will also be more responsive.

FlyLS6

I've been working on my A/F using dyno runs over the last few months. Generally, the operator will "cruise" the car on the dyno while the engine get's to temperature (most of the time the car sits a bit before it gets tied down for the pull). During the "cruise" cycle, I've noticed that the A/F is always lean, relative to that during WOT. I've seen 15, 16, higher frequently in the warm up period, in which the rpm is run in the 2000 - 2500 range.

It freaked me out at first, but the operator told me it's totally normal. Now, all I worry about is WOT, and if you're running a mild/moderate build 12.5 is probably on the rich side. I'm trying to get mine to stabilize in 13 range both before and after the secondaries kick in. If you get the WOT right, it may be that the off WOT and cruise mix is just fine. One thing that will happen is that in cruising mode, just pulling off the throttle the smallest amount, will shoot the A/F into what appears to be a really scary lean condition. If you ever get to dyno your car and the A/F is constantly displayed, you'll see what I mean.

johnt365

Alright, you guys make some good points and I'm glad I posted. Part of my issue is that right around my house there are a lot of hills and stop lights. Once I get to a highway, its a 75 mph flat road.

I do have an automatic and think the torque converter may have been hiding some mild surging. So, I changed the jets from .070 to .071.

What I noticed was that 2k rpm in 3rd around town from light to light is a different a/f ratio from 2k rpm in .67 od at 70 mph on a flat highway. The numbers now are

Idle - 13.2ish
3rd at 2k cruise - 13.7-14.2
4th at 2k cruise - 14.2-15
WOT - 12

WOT is back to being too rich but this is not a race car. I'm gonna hold here and give the poor airhorn a break and focus on smiles per gallon for a while.

Thanks for the feedback.

Shark Racer

Just a note here, just about every wideband controller you buy will report 14.7 for stoich regardless of the fuel you're running. It has no way of knowing.

If you're running pure gas, stoich on a FAST or Innovate will read 14.7. E10, same thing.

So treat the numbers like you're running pure gas and you'll be fine.

If you shoot for an indicated 14.3 you'll be running slightly rich.

John, your numbers sound great. It would be nice if you could get that 3rd gear number up but not at the expense of the 4th gear number. Sounds great to me.

Even the 12:1 WOT is not a big deal. You may be able to lean that out a bit by playing with the secondary hangers. But eff it. 12 is a great number. Enjoy it!

v2racing

You're correct. Stoich is always a lambda of 1. Most AFR gauges are set to run straight gas and do not allow for a conversion for different fuels, so stoich will always be 14.7. I should have included this in my post.

John I think your numbers are pretty good too. I also agree that if you want to lean up WOT, do it on the secondaries. the primaries are such a small percentage of WOT that I would just leave them set for best cruise.

johnt365

Thanks, I will do that. My carb is a 1978 model so from what I can tell, it was originally equipped with K rods. By the time I got it, it had been given 44B rods along the way.

I bought the .044 rods from Cliff Ruggles which are shorter and I guess more like the K rods. The only size primary rods I can find for the K series are .044k so that limits me to jet changes and secondary rods and hanger height.

I am using the CH sec. rods which are .0570 S

Best regards

larrywalk

Going back to the configuration with the 70 jets, I would have been tempted to try 3/4 of a turn CCW of the APT stop screw. This will hold the primary rods a tad higher out of the primary jets at full vacuum and enrichen the cruise mixture slightly while holding the other AFRs constant.

FlyLS6

I think you'd better start reading you plugs. I also think that if you maintain a 12 to 12.5 A/F, you'll see you're running way to rich.

Shark Racer

He's going to run a quarter mile pass, shut the motor off and read the plugs? 12:1 at WOT is fine, if a little safe.

What do you recommend?

diehrd

I should have made videos as I wrestled with the a/f gauge tuning because what is perfect at wot , mid range may not be off idle and any combo in between . I was trying for readings thinking drive ability would be achieved and the reality is that is not the way to approach tuning a carb.

Realize the numbers used on fuel injection DO NOT apply to carbs , it took me a while and a lot of research to understand that and once you know that you can dial the carb in perfectly with the A/F gauge.

As a rule of thumb a carb will run richer then F/I . My guess is a carb can not transition as quickly to throttle movements as F/I which would make sense on why it runs a bit richer.

On my ZZ430 clone with a Holley 800Dp it runs perfect with an idle of 11.7 and cruse of about 12.7 , WOT is around 13.2 . One thing I did notice no matter what carb I used is when you move the throttle you always get a lean spike , very brief but it is there and the trick to tuning is to keep that spike from being to lean a number and to long a period of time.

So use the gauge but use it based on drive ability realizing the 14.7 ideal for F/I but not good for a carb.

Shark Racer

14.7 is fine for cruise with a carb. Idle is where you may run a little richer than expected. My idle is in the mid 13s, any higher and I'd start getting lean spikes.

johnt365

How did I miss this! I have poured over the paper from Lars and the books from Cliff Ruggles and Doug Roe. I never snapped to the beauty of the APT screw. There it is on pg 12 of Cliffs book. Lars even talks about knocking out the plug in the airhorn above the APT for easy access. I assume you could install a plug. What a great idea.

I had totally glossed over this feature of these carbs. The long taper of the rods that Cliff sells finally makes sense to me.

Larry, that paragraph you wrote made it click for me.

diehrd

http://www.enginebuildermag.com/Arti..._equation.aspx

Read through that , every engine is different but just be careful with being to lean.

FlyLS6

I'm sorry, I thought he might be concerned about overall driveability, not just 1/4 mile at a time. AT 12:1 on a street driven car his plugs will be a nice toasty black all over.

Maybe he needs that much fuel flow to keep from detonating in his particular engine, but if that's the case, he's got other problems.

v2racing

If a small block needs to be very rich to feel like it runs right, it usually is because the ignition is not set up correctly. Often this is the case when no vacuum advance is used. Another thing that makes them need richer AFR's is a very loose squish resulting in an inefficient combustion chamber.

A properly set up and tuned engine will run it's best close to stoich (as mentioned, 14.7 on most gauges) in cruise and somewhere between 13.2 and 12.6 to 1 WOT. Idle will vary with intake, cam, and carb combination. Stock engines will idle well between 13.8 and 14.2. An engine with a single plain and a pretty good size cam, for example, will idle better a little richer. More initial ignition timing will allow for a leaner AFR at idle.

Speaking of AFR gauges. Lab grade instrumentation will often let you choose to read in lambda or AFR and switch to different stoich for different fuels.

Shark Racer

You do realize that WOT stands for "wide open throttle", right? He's running at near stoich everywhere else. Not 12:1.

Hence my quarter mile at a time comment.

Many of GM's EFI tunes call for very low 11s WOT and I assure you those plugs do *not* get sooty black.