Re: 1998 Mercruiser 3.0 will only stay running if pumping throttle handle
check the filters, the one on the bottom of the pump and the one at the end of the fuel line entering the carb as well as the one for the water seperator if so equiped. after that make sure ther are no obstructions in the fuel line from the tank and remove the pick up tube from the tank and clean the pickup screen @ the bottom then make sure the antisiphon valve is operating correctly then make sure the tank vent is not obstructed if all checks out and issue still persists make sure your getting a solid 2 shot of fuel into the carb from the venturi cluster just below the air horn of the carb (have someone work the throttle while you look ) with engine off of course after that and issue is still there I would test fuel pressure. If any of these terms aren't ringing a bell I would suggest you purchase a service manual for your rig or request the link on this site for the downloadable version and start reading.
