To get my 1985 Mercedes to start I have to spray starting fluid in the intake. Otherwise engine will take continuous cranking for quite awhile to get engine to start and then rough idling for about one minute holding the gas pedal down before the engine will idle somewhat smoothly. Fuel pump and filter has been changed. Help

look at injectors,

Do a fuel pressure "internal leak test" as outlined in the factory service manual.

Check the cold start injector if you have one of those.

First, however, determine specifically what model is the subject, and if injectors are involved, and what service manual, if any, would apply.

There were at least 10 different models for US delivery alone.