First time tuning with dual channel AF wideband...I have questions

Some background: When I brought my cb450 project to life with stock carbs I didn't know correct tuning and what to look for so when it ran like crap I went the route of new carbs. These larger racing carbs gave me hell so when I went back to stock it was easy to troubleshoot and get it running well.

With the dual channel wideband air/fuel sensors on order I drove the bike daily and racked up about 300 miles with the spark plugs looking darker than I'd like but the bike seemed to be running well; no back firing, idle well, snappy throttle but maxed out at around 75-80mph.

Install: I got the exhaust bungs welded in and made sure there were no exhaust leaks when I put the pipes back on, I used heavy duty clamps on all the slip fittings and exhaust putty around the seams for good measure. Wiring was a breeze but had about 8 extra feet of thick cables that I had to tie wrap under my seat. I calibrated the sensors in 'free air' and started the bike to find that my idle was rich at ~9-10 A/F.

Tuning: For reference Stoich A/F is 14.7 and most naturally aspirated engines are happy at idle in the 12-13 range, ~13.5 for steady 1/2 turn throttle and ~14 for WOT (thats what my researech showed me, if anyone has different numbers I'd like to hear about it) The meter responded to pilot screw adjustments and the bike seemed to idle fine in the high 12s. At any RPM the bike dove back into the 8-9 range..WAY too rich.

I swapped my main jet from 150 to 140 with no difference, then I found some thicker needles from another set of carbs and put those in. Partial throttle is now 10-11 range but I'm getting backfiring on deceleration (confusing since this is a lean condition). The plugs have a clean porcelain look with dark black around the thread rim.

Conclusion: I wanted to get a good baseline with stock carbs before putting the racing carbs back on and tuning with the AF sensors. I'm getting mixed signals from what i've read about the engine (popping on decel) and what the meter is telling me.

Anyone have thoughts or experience?

Popping on decel is not uncommon. It happens on most carbbed engines. You usually don't notice it because stock exhausts help to muffle it. I wouldn't worry about it.

Tune for when the bike is under load and the throttle is off-idle.

Max power is around 12.6:1 and for sustained top end, it may be useful on an aircooled motor to go richer than that to keep it cool.

At idle I would tune it the old fashioned way and see what numbers that equates too.

Transitional i.e. rolling on the throttle should be richer than stoichiometric (11 to 12 range) but cruise on say 1/4 to 1/2 throttle should be able to run closer to 14.7 or even leaner IF your bike is OK with that sort of mixture.

What I learned was that old air cooled bikes and two strokes don't always want to do what the theory says they should do. Air cooled typically seem to want a richer mixture that water cooled.

Backfiring though the exhaust isn't necessarily an issue but on a carbed bike it should be minimal. It's also possible that your exhaust gas has oil and unburned fuel in it and that could be causing higher readings. Our old carbs were never very good at atomizing fuel. What we forget is that fuel burns on the surface of the droplet and with large droplets the fuel is not close to all being burned.

We have seen that on the dyno as high (unburned) HC levels with O2 and CO levels that looked OK or lean. I have no idea how a heated ultra wide sensor deals with those conditions. They are designed for modern fuel systems with ultra small droplets.

And just to add to the confusion, I read that A:F gauges typically read much richer than the real numbers at the rich end of the scale. Given the way a lambda curve looks, that's not surprising.

That was good advice, thanks!

I dropped the mains down from 140 to 130 (originally they were at 150) and the bike takes a few more attempts to start from cold. I got the idle back using the 'old fashioned' method with numbers around 12.8-13.4 and on the throttle the bike really wakes up from previous jetting config. Rolling on throttle gently is 10.3-11.0 cruising at 1/2 throttle gives 10.5 and going up a hill at nearly full throttle gives 14.0.

I'm noticing that engine breaking makes the numbers spike to 16-17 but they quickly settle to normal idle numbers after that. I might go from 130s to 135 just as an added level of safety but other than that I'm pretty happy with the way the bike is behaving. I rode her to work this morning (20 miles) and in low gear acceleration I could feel the front end get light and start to lift. The bike has never wanted to do that before.

Oh btw, I'm running old Honda velocity stacks.