Your question means you aren't fully understanding what is happening. There is no better mixture. Whether you set control at 50% or 40% the car runs exactly the same, unless it ever runs beyond control.

Setting to 50% mean the car can correct equally for deviation of mixture both lean or rich. Try it. Set it to 50% watch the control needle goes rich lean rich lean etc etc. Now richen it to 40%, watch it, needle goes rich lean rich lean etc etc. The actual mixture has not changed as the system compensated for the wrong correction. As long as it is in control (rich lean rich lean) the mixture is the same.

The exception and the reason I prefer 40% is that all dynamic mixture is based on the settings not control. This means that when cold (before control) the car is set richer, sudden accel is based on settings as the system doesn't react that fast. But for constant state it make no difference what you set it too till you run out of range at either 0% or 100%. Any setting that allows control is in effect the same.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician