Boat engine

I have a 1990 omc cobra. 3.0 4 cyl. Changed fuel filter and plugs. Will run for 5 min. then it sputters.

When was the last time you cleaned the spark arrestor? Seems the motor is running ok when cold and it needs a “rich” mixture. As it warms it needs a leaner fuel mix. If the airflow is restricted by the spark arrestor then it cause the sputtering. Is the exhause smoke dark? The is the simplist explanation of a multitude of possibles.

Is this a carburated or fuel injected motor? If it was plugs or plug wires it would run rough when cold and smooth out as it warmed up. Therefore I’d suspect something related to the fuel system. A choke could be hanging up if has a carb. The “cold start” system could be feeding too much fuel if it is fuel injected.