Hot Hard Starting/Rough Idle

My 89 300SE starts immediately everytime and runs smmothly when it's 50 degrees or lower. When the engine is warm on a day above 50, it cranks a long time and when it does start it idles roughly for 5 to 10 seconds, then smooths out and runs fine. My mechanic says that this car has an older(less sophisticated)F.I. system and the hard start with a warm engine is characteristic of this vintage Mercedes. What do you think?