Re: 1978 merc 115hp stumbles
Assuming the carburetors are clean, are the idle speed mixture screws on each carburetor adjusted? You can start with each one turned out 2 CCW turns from a lightly seated position, and then adjust CCW by increments of 1/8 of a turn until it stops stumbling. If they are set too lean (or with the screws too far in), this will make the engine stumble. Outside of that, make sure the fuel pump has been replaced recently and see if the primer bulb loses pressure after warming up. You could have an air leak somewhere in the system.
