Vintage Cars: Hard to Start, vacuum leak, throtle

Question
QUESTION: I have a 1979 450sl with 150,000 miles. It has been working fine. Now it has developed a hard starting problem. I must push the accelerator to the floor and hold it to get it started. It coughs and stumbles and finally starts after about 10 seconds. It does this whether hot or cold. Help!

ANSWER: I would first ensure that your fuel pump is spinning up ( you should be able to hear it) and that the cold start valve is working and if working not leaking.  Leaking injectors  can cause this as well as a sticking throtle plate.  The K jet system is complicated, but easily diagnosed.
Are these original injectors, what is service history?
Once started, does it run well?  Pick up OK
More information you can give the better I can help.  Good luck

---------- FOLLOW-UP ----------

QUESTION: Once started, the car runs well and picks up well. These are original injectors.

Answer
if throtle plate is OK and functions within spec ( push down on it , should have little resistance, feather back up quickly when released) If this is hot or cold issue, prabably not cold start valve, I would lean to injectors or vacuum leak ( providing no binding on throttle plate)