First off, the engine is later than 1954...that's a plus! Johnson made the 18hp starting in 1957. Second, no matter what a local mechanic tells you about how much better todays oils are than "way back then" the engine needs at least a 24:1 fuel/oil mixture to live a good long life. Added benefits of the mixture are better internal sealing, less piston skirt wear, plus numerous other benefits. Todays oils will smoke less than the old oils as well so do it right! You absolutely will not foul plugs on this mixture if your ignition system is in good condition and you have the right carburetor setting in a good functional carburetor. Flooding or leaking carbs can foul plugs as well as weak or cracked coils, dirty points, condensor starting to fail, etc. can give you a misfire similar to that of a fouled plug. Give us a service history of the engine if you can and we can guide you in the right direction.