I have a 1993 70 horse Force outboard with low hours on a 17 foot Maxum Bowrider. It has been very reliable. The last few times out it slowly started to lose power while attemting to increase speed. Fuel line was clean and breathing. Checked plugs(correct plugs that I have been using problem free for years) and all three were fouled with a dark hard oil. Cleaned them and it ran fine for a few minutes then same occured. Engine gets low usage.(At summer camp in Maine, short season.)I have not decarbed,was told WOT does the same.For Oil mix I have been using Penzoil this season. I use 16oz. for 5.5 galls. Any ideas or suggestions would be great. Steve in Danvers MA