Vintage Cars: 1976 450sl, vacuum leak, throtle

Question
the only way to get car to run is to manually operate air intake it will go to any rpm as long as i continue opening plate further but closes as soon as i let it go.why wont it operate on its own

Answer
my first thought is a mjor vacuum leak or a stuck throtle air plate.  Is this a  sudden onset or gradual?
how many miles, any service? I need to know a bit more information, however first and formost look for broken rubber lines, disconnected lines, everything in the car is vacuum dependent and all related
Ron