Wrong There was a 280E with the M110 engine.. A W114 then..A 280E is a 123 chassis, and is fuel injected. Is this a carburated 280C? If it is, it may be the float in the Solex 4 bbl. carb. Mine went bad over time, and became harder and harder to start, would die sometimes, and load up (flood) at idle. I adjusted (lowered) the float level a few times, but that is only a temporary fix. Floats are available and relatively inexpensive, also quite simple to replace. Just be sure you adjust it to specs. Also, check the choke for proper operation before you go any farther in troubleshooting. The choke is primarily electric, and if unplugged or inoperative, the choke will not unchoke properly.
Ahem.mbbuff, I have to rebuff you (pardon the pun) - Australia got w114 cars with the M110 motor, both fuel injected (280E) and carbied (280).
Joelll, sorry I cannot add much more as I had the mis-pleasure of trying to get an M110 going in a W123 and it was a bugger. Vacuum lines, lots of sensors, many things associated with the injection and ignition that can go wrong and are in horrible places to get to. Hopefully Mercmad6.3 will post a response as he got the M110 going for me.
Cheers, Drew in Brisbane