I went to my local parts store to get two Champion J6C plugs for my 1954 5.5HP Seahorse (CD-11) like the manual says and they didn't have them in stock. However, the clerk imformed me that the J8C plug was a better application since it is a little hotter and today's oil is better quality than it was in 1954 when the motor was built. He also showed me this website, which lists the J8C plug as the correct one for the motor:
I'm a little skeptical about putting a different spark plug than that specified by Johnson in my motor. Does anyone have any opinion about switching to a hotter J8C plug? Thanks.