Re: Looking for a year
Nice little motor. The first motor I ever completely rebuilt was a 1955 Fleetwin. The 7518 part of the serial number you posted is the model number.
Your next question will be "where can I get parts for a 1955? I can only find parts for 1956 and newer." Relax, all the tune-up parts are the same for all the 7.5 hp motors from 1954-58.
Use 24:1 TC-W3 outboard motor oil to 87 octane gasoline. Synthetic oil smokes much less and is biodegradable, which is nice since these motors dump a lot of unspent fuel in the water.
Spark plugs are Champion J6C gapped to .030", points gapped to .020", use 90W lower unit gear oil. Check the coils as they are often bad (cracked, sweaty, discoloured) and look to see if there is any water intrusion in the lower unit.
You should replace the impeller. You'll have to remove the powerhead to disconnect the linkage. Sounds involved, but its really quite simple. Check out how to tune-up a 5.5 hp (basically the same motor) over at the following website. Don't worry about the pressure tank thing he mentions. There is great debate on the subject.
http://www.outboard-boat-motor-repair.com/Johnson/default.htm