You will hear it called making oil. The motor runs to cold and can't burn off the condensation. All that excess ends up in the oil pan and Yamahas seem more prone to that. So before anything check your thermostats.
If that is a 28A you HAVE to buy the manual to change the impeller. Also special tools needed. This site has everything you need.
http://www.johnsmcintoshvintagemercuryoutboard.com/
You need an open air spark tester also. Spark needs to jump a minimum of 7/16". Without a manual to tell you what the specs are your DVA test may be useless.
Did you leave the motor in gear when you shut it off? There is a recoil lock on that if you leave the throttle advanced. That motor has the shifter in the tiller handle?
