The 70-72 280SE USA 6 cylinder MFI cars have a speed relay that shuts off the vacuum to the vacuum retard on the distributor at specified RPMs. If the speed relay isn't working then you might not be getting the full ignition advance at higher RPMs.

The Mercedes Factory shop manual lists some very different timing specs for 1972 USA 6 cylinder MFI 280SE cars compared to the 70-71 USA 6 cylinder MFI cars. 72 specs are: 6 degrees ATDC at idle, 36-44 degrees BTDC at 3000 RPM and 40-48 degrees BTDC at 4500 RPM. The speed relay should cancel the vacuum retard at 2500 RPM and give an additional 20 degrees of advance when the retard is canceled. If the speed relay is defective you may not be getting the full timing advance at higher RPMs. That's why you really need a tach that can go higher than 2500RPM so you can verify the timing at higher RPMs.

The speed really was defective on my 1970 and I was never able to get the timing set correctly until i replaced it with a good used relay.

I also am attaching a shot of the specs out of the shop manual just for reference.



John

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