Testing vacuum advance unit
 

My 1973 450sl is getting poor gas mileage about 6mpg and a miss .Wires,cap rotor are new. I replaced my vacuum elements in the trunk and doors and they work fine. I pulled the vacuum line from the distributor and the car runs smoother. I used a mitey on the distributor advance unit and it does not hold vacuum is this the sign of a faulty advance unit . Don't laugh I am not a mechanic just a DIY'er. Any help is appreciated

If it won't hold vacuum it IS defective.

Till you fix it set your timing to 10deg BTDC with all vacuum lines off the distributor.