Found him, and PMed him. Thanks!

The mechanical advance in the distributor can be checked for operation by removing the distributor cap and twisting the rotor clockwise. It should turn a small amount and then spring back when you let it go. As everybody else has said the vacuum retard cell is usually not connected or has a torn diaphragm and no longer works. The vacuum cell retards the timing at idle mainly for emissions purposes. As soon as you open the throttle, the vacuum retard is "turned off" automatically giving it around 10/12 degrees of advance. Then as the engine speeds up the mechanical advance gives it 15 or so more degrees of advance. When the timing is set to the book spec of 5 degrees ATDC and the vacuum retard cell isn't working, you will never get the full amount of timing advance causing the engine to run like a slug. I have seen that many many times on 4.5 engines so it is a fairly common problem. With the vacuum retard cell and the mechanical advance both working perfectly I like to set the timing on 4.5 engines to 0 TDC. This gives it a little more advance than stock since the stock settings were retarded for emissions reasons. If the vacuum retard cell isn't working you can set the timing to 27 to 30 BTDC at 3000RPM. This about 5 to 8 degree over the stock setting of 22 BTDC at 3000RPM. If you set the timing more advanced than the stock setting, you will also have to set the idle speed down as advancing the timing will cause the idle RPM's to go up

Common D-jet problems are usually related to bad injector seals causing vacuum leaks, dirty trigger points inside the ignition distributor, the Aux idle air valve becoming stuck open due to deposits that build up inside, cracks in the rubber hose the goes between the intake manifold and the Manifold pressure sensor on the firewall, and the fuel pressure set to the wrong spec. Fuel pressure spec is 2 bar or roughly 29PSI.

Misadjusted valves and a worn/stretched timing chain will also cause poor MPG and poor performance. Check out this great write-up by Tomguy on checking and replacing the timing chain.

PeachPartsWiki: M116/117 Timing Chain Replacement

This site has the specs and tons of relevant information on diagnosing and tuning the 4.5 M117. Even though this site is for a 73 450SE, that car has the same engine as the 72 280SE 4.5

http://handbook.w116.org/engine_73.htm

John

That's a great method for replacing the timing chain - I might do that just for fun next weekend, I've been dying for a few hours worth of car work! It's so therapeutic.