Will the engine rev in the lower gears? When you go to accelerate, is the car downshifting?

Where is your mixture set when reading the duty cycle?

Take some more notes and post them here, particularly the cars behavior under load in each gear. This will get figured out.

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E