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).