Inliners International Forums General Discussion Engines carb tuning: wideband AFR, plug reading, eyeball..

i'm in the middle of tuning a carb on a moderate performance ancient inline motor. i'm having good success, but a huge disparity between what my wideband AFR meter says, what the plugs look like, and seat of the pants.

carb is a Weber new 38/38, two mechanical 1.5" venturis. it's tuning up 'by the book', no surprises, no vacuum leaks, 5000 mi on the engine, etc. engine built by a pro builder (long story) who did a lot of close and careful work, including valve seats and flow work. it's a trough head and very under-square and low RPM.

webers have a low-speed jet (they call it "idle jet" but it's not) and a high-speed jet. (plus idle mix screws and an air corrector for crossover but that stuff's fine.)

i'm no racer or pro builder, but i've done this a long time, i think i'm ok at reading plugs, i understand the physics and how the carb works (though i'm wrong all the time and not pigheaded about it when i am).

to make a long story short, the 'standard procedures' (sic) all work for me, but i installed an Autometer wideband AFR meter in the dash. never used one to tune before. great tool! i think!

for the last month i've dialed it in with the AFR meter, low speed and high speed/load. i have light throttle level highway cruise 14:1 AFR, and low-speed cruise (throttle just cracked, off idle, to say 1/4th open, 35mph flat and level) around 13.5:1. i go up a size, each, AFR drops (richer) about .5:1. leaner jet, leaner 0.5:1 (approx). all's well! high speed WOT reads about 12.5:1. it all seems great!

i know not to rely 100% on the AFR meter, but since it was new, i wanted to characterize it... so with this experience, i set out to tune via pulling a plug.

* plugs are WHITE! there's no detonation (i've left spark tuning til last, but it's probably under-advanced, for now) or junk, not overheated, just white.

* there's a slight off-idle flatspot, a classic lean bog, and according to everyone's Weber guides, low-speed jet too lean. i know that to be true, also.

* seat of the pants, the gas pedal feels "dry", lean.

so now i begin tuning by hand, ignoring the meter. at one point i end up two jet sizes larger (both low and high speed), the pedal "feels" a bit soft, eg. rich. but i know i'm in the ballpark.

here's the thing:

* the meter says that this jetting is 11:1 AFR! i was expecting it to be much richer than stoich (my rule of thumb, very loosely, was 13 idle, 12.8 WOT, 14 light freeway cruise, taken with a grain of salt).

* the plugs are light neutral grey, like Kodak Neutral Grey.

the "high speed" runs, though brief-ish (5, 6, 7 miles) were on a nicely long uphill freeway at 70 mph and according load, i'd kill the engine at freeway RPMs coast up the ramp pull over and pull a plug. same process for low speed (35 mph in 3rd, feather throttle).

i assume the AFR meter is correct, i guess. idle sets up perfectly by-the-book. throttle is almost completely closed, idle mix only. out 1 turn. lean-idle-drop then back to max RPM, AFR is approx 13.8:1. it "likes" 13.5:1 better, that's what i run.

so i know this is a lot of reading, and splitting of hairs, but what does anyone think of this AFR meter's 11:1 reading vs light plugs?

(for now i'm gonna split the diff on jets, and run one size richer than what the meter says, which is a whitish plug, but the AFR meter says 12:1 or so).

Tom,

Which wideband are you using?

Under cruise - the plugs should be white - most EFI systems will run the engine at 15:1 AFR. If the plugs are not overheating then your fine.

Under full power 12.5 to 12.8 is a good number. That is IF the intake manifold it distributing fuel equally.

The wideband will "average" all the cylinders.

it's an AutoMeter, 6 months old, forget the model and which sensor its using, its a fairly common model. i think i'm overthinking things. everything in the system from tires up is behaving by the book, so i'll err on the slightly-rich side for now, which is one jet size leaner than what i've got, then set the spark map and get some time on it.

modern gasoline formulations seem to burn "whiter", eg. less crud left behind. there's no contamination in the white, the electrode is just fine, so i think i'll trust the meter unless someone else can illuminate the disparity.

this dumb old trough intake Nash head, has perfect fuel distribution, middle and ends. it's short and has interesting anti-reversion wedges in the trough that work. the trough is the least of this engine's limitations.

this is likely my last carburetor. i'm ok dialing them in, well enough for my purposes, but im doing more driving in this car well above sea-level (just did highway 191 in AZ, much over 7000 feet, up to 8800 in a couple spots) and a carb is just stupid for this. this next winter, after driving season, i'll likely go with TBI. i'm seriously considering cooking up my own TBI system. i can do the math and the coding. TBI gets me all the low-hanging fruit. it's 200 hp if i'm lucky.