Hi David,
The Weber only has the circuitry to understand the narrowband input.
Every engines best fuel economy comes at a different figure and the problem you have is getting the best balance between responce and economy.

The YB hates anything under 15:1 and with bad cams it can hate anything leaner than 13.5:1 too so it can all be a little spec specific but in general, 1970's technology low compression tractor engines are ok at lambda 1, but dont like leaner, they lose too much torque.

Some of my closed loop cals run richer than Lambda 1, but non run leaner... it totally depends on the spec.