1973 20' SF - Engine Trouble?

[StrangeBrew]

Thanks. The plug does look the same as the others as far as i can see. This model engine, strangely enough, does not have head gaskets (60 degree V4). Instead this model uses o-rings beween the block and the head...I guess there are semi-circular notches in the head and block which the "o-ring/s" are seated in. Either way, it does sound like I should pull the head and inspect. Perhaps the o-rings are shot? I find it odd that it does sound like it's firing fine on all 4 at idle and underway...I will pull the head and inspect. Could running the motor as is "kill" it?