Originally Posted by A1an
I used to have a 15hp Yamaha that I used on my Orkney Longliner, it was a 100:1 but not on my watch so I used to run it at 50:1, when trolling for a half hour or more it used to start coughing and spluttering as the plugs fouled up. A quick blast at full revs would clear it for another half hour.
Yup. I've a 15 Mariner of the Yamaha ilk. I bought it this season and it took me a while to work out the oil mix as everything on the web said 50. I eventually resorted to asking my 86 year old father who recalled the reason he switched to Yamaha in the mid 80s was that all their sub 40 engines ran on 100 so he didn't need different mixes.
My 15 runs on 100 and gets dirty on 50. All I do now is put a little extra in the tank if the engine is going to run flat out for long periods that day, so in reality I'm probably running 80:1 most of the time which doesn't foul but makes me feel I'm looking after an old engine.