I have a 40 HP evinrude and last week in severe conditions I got water down the gas tank vent and contaminated the fuel supply. I pumped everything out and tried new gas and oil and it ran just fine for awhile. After about two hours of fishing it began to surge at trolling speeds or about 1000 rpm's. It would surge and jump between 600 and 1000 rpm's making it difficult to troll. Runs fine at wot but spits and sputters at low rpms. Would appreciate imput.