I have some trouble with my 1970 OMC V6. I starts fine, runs fine, but will not idle or start when hot. I can start it with some help of the gas and make it idle at 1200rpm or above, but as soon as I let go of the gas and the revs fall to the normal 800 rpm, the engine stalls.
I guess it is a fuel and/or air problem. The engine gets plenty of fuel, so is it getting too much? Or is there an idle control valve that should adjust the airflow when hot, that doesnt work?
I have the same problem with both an inline MerCruiser 165 and an OMC 155 V6. I believe that ethanol is a contributing factor but I do avoid it. Also, fuel blends have changed here in the US and that may change the vapor point of fuel also.
So, here are a few suggestions that have worked to an extent for me. Run the blower to keep engine compartment temps down. Also, mine seem to idle OK in neutral but die as I shift into gear. I richened up the idle a bit which caused it to roughen up a bit. Also, that slowed it down some, so I turned up the idle speed stop screw. That smoothened the idle out some. Both now typically idle without problem but not perfectly smooth. I guess that adjusting the idle to perfect smoothness puts the engine on borderline operation under a load.