With a 71 280SL you may have no solenoids on your injection pump? Could also have just the deceleration solenoid? Anyway, make sure your colds start valve on the intake is functioning correctly. The intake fitting of the valve has a fine screen built in it, that can be clogged. The fuel jet on the CSV can also be clogged or varnished. You might try removing it and hook it up externally to watch it function. If yours is the late "Version 5" (starting aids), then there is no injection pump starting solenoid on your IP. With this version the intake starting solenoid is the only starting device used. It fires at eng. temperature under 95 F. and increases in duration as the engine temperature decreases. The correct function of the intake solenoid is critical on this version.