I recently purchased a 1974 Mercury 115 horse engine which runs eratically and surges. What I have done thus far is: Replaced old steel gas tank which had rust inside and in the gas. Filled tank with fresh gas. Replaced the Plugs, gas filter, checked plug wires continuity. After running a few minutes the number 5 plug has a creamy white substance on the firing end. I am new to outboard engines and could sure use some troubleshooting advice.
