ohiovet

I just put an spare wide band oxygen sensor on my 62 340HP, AFB car and it confirmed what I knew all along that it is running rich.
I adjusted the idle AFR and it is fine right around 14.7.
But when accelerating it fluctuates between 11 and 14 which is ok but without any big load it does not lean out. At a easy cruise I would like it around 16 or 17 AFR.
So, what should I try first?
Metering rod springs are the easiest.
Metering rods are also easy to change.
Or last the main jets.
Any advice will be appreciated.
Bruce B

jim lockwood

Bruce,

At steady hiway cruise, you probably shouldn't go any more lean than about 15:1. You don't know how uniform the mixture distribution is and you need some headroom to prevent any one cylinder from running too lean. 16:1 and 17:1 are definitely too lean.

As far as the dithering between 11:1 and 14:1 .... well, I'm not a carburetor expert but I have a hunch the metering rods are hunting up and down as you accelerate.

Your idle mixture is more lean than what I would expect. My fuel injected engines idle the best with mixtures closer to 14:1. However, the "best" idle AFR is whatever makes the engine happy; so if your engine is happy at 14.7:1, so be it.

Jim

ohiovet

Jim,
I will check out what color springs I have on the metering rod pistons and make a change and see what happens.
I will also take your advice on the cruise AFR.
Thanks,
Bruce B

SWCDuke

There's no way the engine will run that lean; 16-17:1 is at or beyond the lean ignitibility limit. FI engines may be able to run as lean as 15.5:1 at cruise because they have more even fuel distribution than carbureted engines. A cruise lean condition can cause hesitation and poor response to small throttle increases. You might be able to cruise as lean as about 14.5:1.

Pre-emission engines usually idle best at about 12.5 especially with high overlap cams. Modern emission controlled engines idle at stoic. but they have very even fuel distribution and low overlap cams. High overlap cams have much greater exhaust gas dilution at idle, which requires a rich mixture to ensure consistent light off of the charge.

In normal driving the ratio will vary widely because every time you increase throttle, manifold vacuum may drop sufficiently for a second or so to allow the metering rods to lift to the power position. Plus there's a brief charge of fuel from the accelerator pump. Test cruise ratio at constant speed on level road.

At WOT from low to high revs on carbureted engines, the low rev mixture will usually be rich if the high rev mixture is in the proper range, which should be no leaner than 13.5:1 at peak revs.

Also, wide band O2 sensors must have very fast response time due to the highly varying ratio during normal driving.

If you are looking for better fuel economy, install a single point distributor with an aggressive centrifugal curve and a properly selected full time vacuum advance. With the dual point, install lighter centrifugal advance springs to bring the centrifugal in as quickly as possible and run as much initial advance as the engine can tolerate - up to 40 deg. total WOT advance.

Duke

cardo0

I think your A/F numbers look pretty good right now. Idle at 14.7 is norm for EFI in closed loop and should be used for cruise also. Theres not much to gain in leaning below 14.7 for cruise though small savings in fuel can be found its difficult to find the lean "window" with EFI and a dyno let alone a carb. For ex all OEM maps are 14.7:1 at cruise but they do have emissions concerns also.

Oh u can run it leaner as i got to experience 18:1 on dyno test car (not at WOT but part throttle only) but the loss in torque will require larger throttle opening for the same load and take more fuel for the same power.

Your transition to WOT should be near 13:1 so the 11:1 number is somewhat low (rich) - but u want it richer than 14.7 for max power and exh temp control. Consensus for WOT max pwr is 13.2:1 but no 2 cars are the same and its your baby.

I recall the Carter/AFB has reputation for consistent operation and holding a tune once setup. If its to rich on acceleration it could be that air vlv/secondary vlv is sticking. There is no adj for the sec vlv (flap) on the AFB so to change its operation (opening rate) u have to add or remove weight. In your case it looks like it maybe slow and u may want to take some material off the counter weights. Problem if u take off too much weight or that wasnt the problem then u have to add weight(s) to return to norm. Maybe a lot of work.

Hope this can help.

65tripleblack

Idle a/f ratio should be around 14.0-14.5 with your cam.
Steady state cruise a/f ratio should be around 14.0 -14.5 with your cam.
WOT a/f ratio should start at about 11.5:1 (@ around 1500 RPM) and steadily increase to about 13:1 (@ around 6000 RPM).

Faslane

You will need adjustable air bleeds to tune that out correctly

jim lockwood

What no one has stopped to ask (and I'm guilty also) is what fuel are you using.

It matters.

If you are using Ethanol-enhanced fuel, your cruise and WOT mixtures need to be about 4% richer than if you are running non-Ethanol fuel.

The numbers I cited are what I would shoot for if using non-Ethanol fuel.

BTW, the design spec cruise mixture for Rochester FI is 15.5:1. At this AFR the engine runs fine.

Jim

ohiovet

Jim,
I usually use 93 Shell with ethanol.
The car has always ran well with this gas.
Bruce B

jim lockwood

In that case, Bruce, the richer numbers suggested by others are more appropriate than my recommendation...... unless your wide band measuring equipment is from Innovate Motorsports.

If it is, that's a special case and my recommendation is valid, regardless of the fuel you use.

Jim

ohiovet

Jim,
I have a Innovate MTXL wideband with a Bosch O2 sensor.
I used it with my Megasquirt ECU but am now changing to a FAST EFI 2.0 system.
What is different about the Innovate compared to others?
Thanks,
Bruce B

jim lockwood

Fair question, Bruce. Long explanation follows:

The Innovate equipment measures a property called lambda which has a value of 1.00 when the air/fuel ratio is at its stoichiometric value for the fuel you are using. It doesn't matter whether you are burning pure gasoline or gasoline with ethanol or kerosene or E85 (bad idea). At the stoic ratio, lambda will be 1.00.

To display a number humans recognize, Innovate multiplies the measured value of lambda by a constant which we understand to be the stoic ratio of gasoline, 14.7. (You can change this constant, but 14.7 is the default.)

So, if you use pure gasoline and tune your AFB for stoic, your gauge will display 14.7. If you use E85 (don't) and you tune your AFB for stoic, the gauge will still read 14.7 even though the stoic ratio for E85 and the actual AFR is really 9.8:1.

In your case, with ethanol enhanced fuel, stoic is 14.1:1 but your AFR gauge, which is really tracking lambda, remember, will still report the AFR as if it were 14.7:1.

So, bottom line, if you adjust your AFB to the richer numbers suggested by others and you use the default multiplier of 14.7 on your display, your actual AFR will be approaching stupid rich. You should use the numbers I suggested.

Three final thoughts:

1. Dial in the AFR of your carburetor with the fuel you plan to use. If you use fuel with ethanol while adjusting the AFR, and then find a source of "pure gas" for day-to-day driving, your day-to-day AFR will be wrong.

2. If you possibly can, carry a laptop computer with you and record your test drives for later, off-line review. At review time, you'll see things you didn't realize were happening because you couldn't watch the gauge full time.

3. Air/fuel equipment from other manufacturers may take a similarly simplistic approach to calculating air/fuel ratios as taken by Innovate. Innovate equipment is what I use to adjust air/fuel on Rochester Fuel Injection and it's the only equipment with which I have experience.

Good luck,

Jim

tbarb

I would like to add my feelings about tuning the AFB if that's what you are using. There are very few things to change on the AFB, main jets, metering rods etc. Keep in mind these changes effect mostly the main circuit at RPM's above 2500.

Don't try to tune the engine idle speed to a certain A/F ratio, the idle circuit tuning is about the A/F ratio while the engine is running on the transfer slot (up to approx 2500 RPM) and the emulsion screws simply fine tune the idle for best stable idle.

For the idle circuit, the second idle air bleed in the venturi cluster can be taped for brass set screws and you can enrich or lean to see what that gets you in the low speed cruise. It's probably best to leave the idle feed restriction alone.

cardo0

Good point and with todays pump fuels lambda is a better target. Problem is that using a carb u dont have compensation for the change in fuel from different sources/different blends (or temp/air density) like u would with an EFI. But still, who tests their fuel before tuning? Anyways i still love carbs as the fuel mix is not a cruical to power as the timing. U still have a max brake torque for the fuel mix (A/F) but not nearly as sensitive as ign timing.

Thx for sharing.

devildog

Bruce,
I use Innovate A/F on my 61 with the DART 400 engine. I increased the flow on my old 2 x 4 WCFB carb intake to flow enough air to make near 500hp.

Do you have the Bosch O2 in a bung or checking tail pipe? I have a bung in each pipe and switch O2 sensor periodically

As John said, no way are you at 16-17 A/F; I would guess that you have an exhaust leak up stream of the O2 and are sucking air into the system.

I run at 38° advance @2500 rpm. If I change metering rods and get cruise A/F above 14.8; it is too lean...I get some surging and bucking (too lean???). So, I target to get 14.5 A/F.

My old WCFB's are the same basic design as your AFB, and I have good success swapping metering rods to change A/F ratio. It is quite sensitive with the small rear barrels on my rear WCFB.

Just ± .002 " diameter on the metering rods makes difference

Joe

Faslane

Drill the air bleed one size, i change 2-3 from size swap up or down.
IMO your #s are fine, i have a older wideband and i don't think it even goes to 18.
I built a blow thru turbo so I got schooled on the carbs. I could not get it like I wanted till I adjusted air bleeds, they affect it tremendously
Lane

jim lockwood

Hmmmm...... Joe, I think you might have some uneven mixture distribution happening. I've run my '60 fuelie with cruise air/fuel slightly over 16:1 (not on purpose!) and it didn't surge, buck, stumble, etc. Were it not for the gauge, I wouldn't have known the mixture was lean.

Jim

GTOguy

What these gents said, big time. As a CA smog tech for the past 34 years, I've had some experience with emission gas analysis. 14.7 is the sweet spot for an internal combustion engine at idle or cruise. If you can get that with a carbureted, non-computerized car, you are doing very, very well indeed. Most run a bit richer, at least with 100% gas. Leaner than 14.7:1, you enter high combustion temperature territory: pinging, burnt pistons/valves, lean misfire, and very high NOx emissions. Chrysler tried this in the late '70's with it's Lean Burn system (remember the PCM that was attached to the air cleaner?) and it didn't work out well. The cars ran roughly, hesitated, ran hot, ran on, etc. With these older Corvettes with iron heads and pretty high compression, on today's crap fuel, you want to be a hair richer than 14.7:1.

ohiovet

Joe I have a Bosch wide band sensor in 1 exhaust about 12 inches from the exhaust manifold.
I would like to install another one on the drivers side which I will do someday.
I need to pull the metering rods and the springs and make one change to see where the AFR goes.
Thanks for the info.
Bruce B

devildog

Jim, I agree. Since I took 400 cc of aluminum out of my 8 runners in that old intake with a rotary wood rasp and did not flow tested each runner, I am sure there are some mismatched flows. Also, since the back 2 barrels on the rear WCFB have a long distance to feed the front cylinders, probably some imbalance associated with that.

Anyway, it is very drivable and can surprise most any C-5, C-6 between 20 and 80 mph.

Joe