I think you had it right the first time.

My understanding is that as duty cycle increases, the valve is open more and more fuel is diverted from the FD chamber. So as you richen the mix, the O2 sensor voltage goes up and the computer tries to lean it by keeping the valve open and diverting some more fuel.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe