A few other thoughts if all of the above does not fix the issue.
Those are 12 year old engines with only 400 hours which tells me that they are not used very much. If the plugs have not been changed or at least pulled and checked recently, I would do that. If I did not know the history of the plugs, I would change them out. Use NGK IZFR6F11 only. If they have never been changed, I suspect you will have a difficult time getting them out and may need a professional to do it, or alternatively, spend a lot of time with some PB Blaster.
Also, if the engines have been "babied" a lot, or used for a lot for trolling, I suspect they could be carboned up a good bit, so a concentrated decarbonizing procedure should be considered using something like Yamalube Ring Free or similar product made specifically for that purpose. This should free up any stuck rings and carboned up valves.
Finally, if the engines were laid up for a long period of time it is possible that the valve stem seals have become hard and are allowing oil to seep into the cylinders. However, if this were the case, then you would ost likely be consuming oil rather consistently, which you did not mention.