Spark plug question

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Mike Redmond
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Joined: 21 Feb 2009, 15:51

Spark plug question

Post by Mike Redmond » 09 Oct 2016, 19:14

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

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Joined: 11 Jan 2012, 10:26
Location: Albion, IL

Spark plug question

Post by acwd » 11 Oct 2016, 08:36

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.


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Spark plug question

Post by Pappy » 11 Oct 2016, 18:53

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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