Hi all,
I know not an Offshore issue but I have one of these too. I have a 50hp Johnson 2 Stroke externally oil injected. Starts and idles fine, revs with no issues. Under a load it runs fine until over 4000 rpm at that point it starts surge and fall of continuosly does not die and will do this all day. It will return to idle just fine. I have changed plugs, fuel filter, checked for all leaks out and in (air) nothing... Any thoughts? I don't do outboards

What year is the engine? How did you check air leaks? Normally one would use a piece of clear fuel line with a tee and a vacuum gauge. This is placed in the fuel line at all the junctions one at a time looking for air bubbles and under 4" of vacuum. Does it have a built in tank or a portable tank. You could try to run it on a portable tank. If it is a built it tank pull the siphon tube and check for a plugged screen, elbow or anti siphon valve. If this all checks out then it is time to pull the carbs and give them a cleaning. Your sync & link is sensitive as well on these so make sure that is done and done correctly.