spark plug question

[R. Bushie]

Hi, Happy New Year ALL!! I have a 1958 35 hp Evinrude (model # 25936)and I'm a bit confused with spark plug selection. My manual calls for J4J (J4C?), but I'm using J6C's. While brousing the Champion catalog at a local store it listed J8C for the 58 35 hp. From experience what is the best plug for this motor? Also what is the main difference between these 3 plugs? (J4C,J6C,J8C).

 

[Joe Reeves]

Re: spark plug question

The difference is the heat range rating (4,6,8).<br /><br />The J8C is too hot and will result in piston damage. If the engine runs slightly better with the J6C rather than the J4C (either one acceptable), go accordingly.

 

[Paul Moir]

Re: spark plug question

Happy new year, Robert!<br /><br />The difference is the heat range. The J6C runs the hottest so it burns off carbon that would otherwise foul it. J4C runs cooler. <br /><br />The J8C runs far too hot for these engines, and will destroy them in time. The plug gets so hot that preignition can easily occur (originating at the plug) which will erode metal off the top of the piston. The Champion catalog is in serious error here. And it's not just your engine but many older Johnny/Rudes as of the latest catalog. Follow the current BRP guide which still states J4C.<br /><br />But between J4C and J6C it's really up to you. If you troll a lot then the J6C will burn off carbon better preventing plug fouling, giving you a better long running idle. The J4C is good for general purpose boating, heavy loads and high speed. It offers an extra margin of safety against pre-igntion. There are other variables that you might want a hotter or cooler plug for like altitude (Apparently, we don't have that here. ) and proping.<br /><br />All the JxJ plugs were replaced with JxC plugs. The difference? The shape of the ground electrode to something more modern. Makes no real difference to us. The J prefix just indicates the thread size & depth.<br /><br />EDIT: Beaten again!