The duty cycle should drop when you give the car throttle (air).

Some people say the sweet spot is anywhere from 45% -70%. Play with it and see if you get any improvement. But if it were me I would solve that stalling problem first. When does it stall?

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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