Could be fuel system related. Carb. Lines. Pump. Fuel itself. Moisture/water/contamination. Or could be ignition related. Spark loss, weak spark, timing related perhaps. Could also be engine related-e.g. valve clearance. Or, could it be prop/rigging issues? At what RPM does the engine start cutting out? Does it do it in neutral, or is it load-related only?
So many unknowns. What has been done previously. What was checked. In cases like these where there is no clear-cut problem, I generally try to isolate it to a particular system on the outboard and then further narrow it down in that system. Like if I know it's fuel system related, I'll isolate it by eliminating the easy stuff first. Fuel flow test. Rig up a test tank connected directly to the carburetors, gravity fed (the tank will have to be higher than the carburetors). All you need is an idle or just a little above idle. Remove fuel line from pump that feeds the carbs and put the end of it in a jar or can, just something to catch fuel. If there is a flow problem, you'll see it right away. It'll either trickle or it'll flow real good at first then peter out.
If it's a spark problem, it's a little tougher to find. Usually it either sparks or it don't.
If it's an engine related problem, check the valve clearance first. You're looking for tight valves. Do it by the book; not the way you learned to check and adjust them on an old car...because they're totally different, very similar to motorcycles.
how old is the fuel? If more than a couple weeks old, dump it and start over with fresh. 4 strokes hate fuel that has any age to it due to the tiny passages in the carbs. Just went through this on my 25, and changed stuff that didn't need to be changed. All it was, 3 week old fuel that was just nasty. I noticed the mower wasn't running right either and that was the dead giveaway since both were from the same station, same day. Supposed "non-ethanol" too but I ran out of test strips to check.
just a couple random thoughts, FWIW.
