Peter -

This engine is pre-EHA. The on/off ratio of the frequency valve is controlled by the O2 sensor via the lambda controller. You are supposed set the on-off ratio to 45% with the mixture adjustment and that should give you a good CO reading. If everything is working correctly, the only way to make the engine run richer or leaner is to adjust the mixture beyond the system's ability to correct (or disconnect it).

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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