1977 280SE. What is the correct timing setting? I have 2 3rd party manuals that disagree. The car has no air pump or EGR valve even tho it is a calif car. One book says disconnect vacuum from distributor and set at 2 degrees BTDC. The other says to leave the vacuum connected and set at TDC. Both agree at 800RPM for idle. Tried both and car runs rough at idle. New plugs new wires and new distributor and cap.