As I said, over a quick learning curve my outboards started to perform much better. The first experience of fouling was when another mechanic mistakenly put in some QL77J4C spark plugs. As many know that plug was specified by OMC for most motors after 1977 until mid 1990s, so it was probably an educated guess, that unfortunately was wrong. Those spark plugs did not last the week. Way too cold of spark plug for the 1974-1976 series IMO.
I then went to the specified champion spark plug (UL81C) and within a month or so I was replacing those. I then proceeded to ensure my rich lean was set right, my T-stat worked properly and the motor was properly decarbed, so when I switched to the NGK plugs (B7HS) and stopped trolling with it, things got a lot better for it. Then a couple friends borrowed the 15Hp and trolled for about 3 hrs and the next day, sure enough, I had one fouled spark plug. I don't know how old those spark plugs were before the trolling but I doubt they were more then 2 years old.
So that is when I stopped all trolling with it and switched the 15Hp to NGK B6HS spark plugs and I have never fouled a spark plug in over 15 years. The motor starts on the first pull and pins me to my seat when I open her up WOT on my 14 foot boat.