I have changed all fuel filters, rebuilt the fuel pump, done the Seafoam decarb procedure from the FAQ and changed spark plugs. At any given RPM, 3000 for example, RPM will surge up and down by 2-400 every few seconds. When I pull the plugs, they all look the same, coated with blackened oil, and black around the electrodes. This is a 2L V6 135HP 1994 carburated Mariner. What should I try next? TIA<br /><br />George