I have a 84 Evinrude XP100 V4.<br />Mod: E100STLCEM Ser: R 08160584<br /><br />When I checked the spark plugs after a day out, One of the plugs had its gap completely closed.<br /><br />So I replaced it and tried again with a new plug, and again it closed.<br /><br />I am assuming the piston is...