Or you can work the poorman's spark test holding an ignition wire (with rubber handled pliers) unscrewed from the plug resistor cap about 1/4" from the cyl head while somebody else cranks the engine. Spark between the wire and cyl head oughta be BLUE...... if it's orange then you've got ignition spark problems, could be internal corrosion of condernser or coil.

Lesser known fact: crummy condenser ($5 part) can prevent the venerable old 6cyl Haeckflosse/Benz from starting.

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'80 300SD/ w116
'79 240D 4-spd
'71 750cc Guzzi

previously owned:

'83 240D 4-spd
'77 280SEL 4-spd
'74 280/8
'72 250/8
'65 220Sb 4-spd
'63 220Sb 4-spd
'63 190c 4-spd
'61 220Sb 4-spd
'60 190b 4-spd