It sounds like you are lean at anything other than idle or WOT. There is a potentiometer ("pot") in the mixture control unit that tells the brain how much enrichment to provide. There is also a three position mechanical throttle switch (idle, WOT, all other). I think your pot may have gone south and you are OK at WOT because when the brain gets the signal from the switch, it overrides the pot's signal.

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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