I once decided to fix up a 4hp johnson outboard that I got as part of a trade. Figured it'd be a good way to learn about motors, and cheaper than taking it to a shop. Bought a book about OMC repair and the service manual for that model, and jumped it.
I ended up replacing the head gasket, fuel pump, points and condensers, water pump, and rebuilding the carb. Not a lot of money, but a considerable investment in time seeing as I had never done anything like that before. Did not want to run. Couldn't figure it out. Sold it to a buddy for what I had in the thing in parts.
He called me the next day. The spark plug wire had rubbed against the flywheel and was shorting out. He replaced it and has been running that stupid thing for 5 years. Just puts oil and gas in it. I'm still mad at myself, but I learned a lot about motors. Mainly, that I don't enjoy working on them and should make full usage of warranties and marine shops, but it's nice to have a basic knowledge.