Spark plug question

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Mike Redmond

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Tinkering around that new to me 1970 20 hp johnson,want to replace plugs,it has a set of QL77JC4 on it,been looking on the net what were the originals ans I get was J4C another says RJ8C ????getting confused?seems to be a mark on top of the pistons when I shine a light in the hole are them QL77JC4 the wrong plug or are they the replacement for the J4C ...can someone tell me what is the diff.between them
 
you can go with a Autolite 295 or NGK B6S or Champion J8C. RJ8C is a resistor plug. The QL77J4C was designed for the CDI ignition system and had a longer reach on the plug thats where the shiny spot on the piston is coming from. I run the NGK plugs in my older stuff and dont have problems with them.

Steve
 
Think we just answered an identical question to this a while ago.
The J8C is too hot for that engine. It calls for a J4C as the primary plug for it.
Usually that means you can substitute a J6C unless you do a bunch of WOT running.
The engine was designed around Champion plugs and the Champion plugs seem to do the best job in these engines.
 

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