Re: 1978 evinrude 55hp 2 cyl
Welcome,if you read the stickies at the top of the psage it will help you understand prop selection.
More than likely you have an engine problem.It may be operating on one cylinder.
You wouldn't be the first to not realize except for the poor performance.
If you carefully feel the plugs after a minute or so run one will be quite hot.
If you pull the plugs one will likely be wet.You could carefully unhook and ground one plug wire at a time and test run.You need to ground the loose wire to protect the ignition.
You can also check that the throttle opens all the way and the stator advances as it shouild.
As far as prop selection It depends on the boat and typical load.
The motor came with a 12 1/4"(diameter X 15"pitch) prop when new. With 4 other sizes depending on the application.
