I have a 72' 6 hp johnson OB, never had an issue with it all of the years I've owned it.

When it is engaged in forward or reverse, the lower cylinder fills with water, it will idle smooth all day in neutral, but I get 100 yards from the dock, and the lower cylinder dies. Could this be a simple head gasket? >

The head gasket is where I would start.  

Might want to sure the spark plug is tight also.  I'm not familiar with that motor but many of the old Johnsons have a two piece head where the water jacket cover is bolted onto the head and if the plug is not tight they will let water get into the cylinder.

The head gasket is where I would start.

Might want to sure the spark plug is tight also. I'm not familiar with that motor but many of the old Johnsons have a two piece head where the water jacket cover is bolted onto the head and if the plug is not tight they will let water get into the cylinder.

thanks, the first thing I changed was the plugs, I thought 1 went bad. I'll try the head gasket, if nothing else, I can get a good look to see how the internals are doing.