You can turn it many revoultions.

Oversimplification of the day: Before tweaking the modulator make sure the rest of the vacuum system is sending it a proper vacuum signal. 15 mmHG or so at idle, diminishing to 0 at full throttle.

Make sure you have your bowden cable and your throttle linkage adjusted correctly. There is an article on this in the FSM.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car