AFM meter reads lean but plugs read rich

[John1976]

I’ve search the ENTIRE web an I’m at a loss…

Issue: Poor gas mileage, rich gas smell, rough idle not resolved by adjusting idle mixture.

The Bus:
I have a 1976 T2 with a stock Type 4 engine with dual Weber 44’s .
The valves are adjusted, the dwell is within speck.
The timing is set at 29 BTDC @2500-3000 and at 10BTDC @idle (perhaps a little hot but we had crappy gas (no lead, plenty of ethanol and god knows what given the coming winter season).
I have new plugs, cap, rotor and wires.
There is a good blue/white spark when wires are grounded before the plugs. The fuel pressure regulator is set at 2.8 psi using a bypass regulator that shunts the excess fuel back to the tank (as opposed to simply fighting the pump).
The jets are cleaned (ultrasonicated; mains are 135, f11, 175 and idle is 60)
I run 87 and/or 89 octane
The floats are set at 12mm (I split the baby between Webber’s suggestion or 14mm and the Samba consensus of 10.5mm)
My gas mileage is around 10.5 mpg, which is much worse than previous performance.
I live in NJ (probably 100-200 ft above sea level)
I have 8 cats and I’m a Sagittarius…

I noticed a change (gas mileage) after the last tuneup (so I rechecked the spec’s as reported above).

The issue is as follows:
The van idles rough and smells rich. The plugs under each card all read rich (dry soot). I picked up a AFM gauge to help tune as I am a novice and haven’t calibrated my sense of smell/sound to the point where I am confident enough to dial in the carbs. The meter reads VERY lean (20-30 air-fuel) but the plugs and tail pipe are full of soot. I checked the meter on my wife’s Audi at it read approx 15 so the meter is good.

I understand that very rich can present as lean so I trust the plugs.

I have followed the Webber set-up directions (idle mixture 1.5 turns out +/-0.5, idle speed 0.5 turns in from initial contact’ and carbs synced/balanced).

I have used a can of carb cleaner searching for a Vaccum leak. I isolated the brake servo but still have a steady idle.

I can get it to idle (rough) at 850-900rpm with a lot of adjusting, and probably some cheating with the idle speed screws but it smells rich.

What am I missing??? Thanks in advance

[airschooled]

"Idle" jets are a misnomer. We should say "pilot" jet because that jet affects all throttle conditions up to about 2,500 RPM when the main jets come in.

A) Your pilot jet is on the rich size for a 2L. I typically start with a 55. 50 is not uncommon on stock camshafts.

B) Your air jet seems a bit rich; 200 is my starting point for 2L engines.

Lean doesn't show up as rich, lean shows up as a misfire which smells rich when the fuel doesn't burn. I think you're actually too rich.

Robbie
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[Wildthings]

A misfire will show up as lean, as it leaves unburnt O2 in the exhaust stream.

[mikedjames]

I have also found that running very rich so that soot builds up in the exhaust makes the lambda sensor indicate lean. Like if it sits around 11:1 or richer, eventually the meter reads leaner and leaner.

For instance leaving the wire off the choke heater on my progressive and then driving around town. If I take it for a high speed run, the choke has less effect.

It may recover if run in a leaner environment, but quite often it has been time to buy a new sensor, and rapidly make the jetting leaner.
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1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[John1976]

Thanks for the timely replies. I’ll order the ‘jets’ tonight. Cheers

[SGKent]

[raygreenwood]

It's also just as probable that you are seeing oxygen contamination from the tailpipe. Where is your sensor mounted?

Ray

[Nitramrebrab72]

Are the new leads going to the correct cylinders ?

[metahacker]

Sounds like you have an exhaust leak, that will make you see ~25 AFR

Motor can't run that rich, not without a stratified charge combustion (e.g. CVCC or GDI) and tons of ignition timing.. so it's definitely telling you there is extra outside gas being introduced to the exhaust before it hits the sensor.

Fix that and the O2 will probably tell you what you already know, that you're rich, and help you get it not so rich.
Sounds like you might be in the 9-10:1 scrooge mcduck party zone
Good that you are addressing it, piston rings don't last under those conditions

Narrow band or wide band? I assume WB if you are seeing #s like 20-30

You are on the path to major victory, just some hurdles to go

Bravo for going straight at it with the oxygen sensor
In theory it will be paint by numbers after it works

[John1976]

Thanks all for the suggestions. Here are some replies…

SGKent: I’ll do a cylinder balance test tomorrow. I thought that each cylinder was responding to changes in their idle mixture screw but I had planned to check the compression of each cylinder as well. The Cylinder balance test looks like a more analytical approach. Will do it tomorrow when I’m back with the bus. Thanks.

Ray: The Wideband AFM is the portable type from RayTech Automotive. The sensor screws into a bung on a short pipe that is placed into the tail pipe and clamped in place. It seemed to read accurately on my wife’s Audi so I think it should be close to accurate.

Nitramrebrab72: Yes. The wires are correct.

Meta hacker: Yes, it is a wide band sensor. I will search for an exhaust leak tomorrow as well. I will tack off the bottom tins and take a good look/listen under the heads as well.

Stay tuned and thanks again! Cheers

[raygreenwood]

[timvw7476]

over fueling from a questionable fuel regulator? Overwhelming the floats.
or you've got the wrong rotor/cap combination causing misfires.

[Wildthings]

[John1976]

Sorry for the late response/update- had some unexpected travel and needed to order parts.

First, i checked the compression and all four cylinders were between 115 and 125 or so. I also took a hard look at ‘the questionable’ fuel pressure regulator. It delivers a steady 2.4 where I now have it set. This doesn’t change from idle to revving up to 3500 rpm w/o a load, and it maintains this with some slight variation (2.2-2.4 or so) under load. Therefore, I think it behaves as designed but it may be an overkill set up.

Secondly, I scrapped the O2 meter for the time being. I changed the jets too.
Originally i was running 135/f11/175 with 0.6 idle jets. Now i am using 150/f11/200 with 0.55 mains. I dialed in the carburetors by sound (and smell to a degree). It Idles at 850-875 or so w/o adjusting the idle speed screw past the ‘half turn in from contact’ bench setting.

The bus runs noticeably quieter. I no longer hear the gargling sounds from those large Webers whiles driving. It sounds much smoother. I am in the process of determining the gas mileage as that seems like the best macro data point to assess the efficiency changes. It feels and smells much better but as they say ‘to measure is to know’.

Regarding the o2 sensor, I am fitting it to a long pipe to test if placing it deeper in the exhaust flow makes a significant difference. I think I’ll need to heat the pipe with a heat gun as well. I am also ready to fire up my super beetle and will test that system w/ and w/o the pipe as a control system. Stay tuned.
Cheers

[SGKent]

generally on Webers, three air correction sizes (marked 5 different each) are equal to one main (marked 5 different each). Going from say a main of 125 to 130 would be one size. Going 125 to a fractional 127.5 would be 1/2 a size. Going UP three sizes on an air correction jet would be the same as going up one size on a main jet, because less air bleeds of pressure at the main. The air correction jets are used to change the slope of the mixture VS RPM.

Changing the mains changes the base line reading, and changing air correction changes the slope of the line vs RPM.
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[Nitramrebrab72]

[Nitramrebrab72]

What number plugs are you running? Too cold a number will not clean and will look rich. Also the AFM relies on a good spark at the correct time it does not measure the Air to Fuel ratio that would mean calculating the mass of the air vs the weight of the petrol. It only predicts what it believes to be by analysing the exhaust gases and it has no choice but to assume the explosion takes place correctly and at the right moment.

[John1976]

OK, I re-jetted as described above and have been using the bus as my daily driver through the fall and spring. Runs great. Dial in a steady fuel pressure at around 2.7spi. Getting about 14.5 mpg. I’m calling it good! Thanks for all the advice.
Cheers

[Wildthings]

If the 14.5 gas mileage is just stop and go city driving then it is okay, but if that is highway mileage you should be able to improve on it. Try rejetting a step in either direction to see what that does to your gas mileage.

[John1976]

Thanks Wildthings- It’s all local roads as a daily driver for someone who is retired.