3L mercruiser i/o

by Cheryl
(Oc, md)

After taking boat out of winter storage, I attached hose and boat ears turned on water and started engine. It runs if above 3000rpm, but anything less than that it sputters shakes the boat and then shuts off immediately when you go to put it in neutral. We checked 4spark plugs and they are ok. Then we bought heet to dry gas and added additional 7gallons of fuel. It still did same thing. Now we siphoned the bottom of tank but dont see much water. What else could be wrong? Please help!