how to read a sparkplug?

[jwilkey84]

I finally got my 20hp johnson to running by cleaning the points up a little. But when I shut it off I took the plugs out to take a look and the top one was black and oily, but the bottom one was black and dry. What does/could this mean? Also the manual calls for a champion J4J plug, and mine has a J8C, will J8C's be ok?

 

[ezeke]

Re: how to read a sparkplug?

Your motor started life using a Champion J4J which is replaced by the J4C. An 8 is way to hot a plug for your motor. Most of us use the Boat Setup chart:

http://www.boatsetup.com/SparkPlugChart.pdf

Now that BRP is listing the parts sales directly, you can also get the correct sparkplugs by looking at the cylinder and crankcase diagrams on their sales charts: http://shop2.evinrude.com/ext/index.aspx?s1=382861efdec7bb315acfdcfac9525765

 

[F_R]

Re: how to read a sparkplug?

Your 1972 year does that normally. Here's why: Earlier years dumped fuel overboard. That was fuel that had settled out of the air charge as it goes through the crankcase. So around 1972 in an effort to clean up their act and stop dumping fuel, they ran it into the upper cylinder to be burned---supposedly. You will see the hose going to the upper bypass port cover. Well, it was a nice try, but it made the upper cylinder run too rich, which is why your plug is gloppy. It also made them idle lousy compared to the older motors.

 

[jwilkey84]

Re: how to read a sparkplug?

I really appreciate the history that comes with the wisdom of this forum. So if I understand the top cylinder gets more fuel than the bottom one? But should the bottom plug be dry and black, shouldn't it be a little oily? I mean this thing was dryer than chalk and black as coal, after just a few minutes of running in a barrel, as we currently have about 14" of ice on the water here in Iowa

 

[Silvertip]

Re: how to read a sparkplug?

Because you were not running it very hard in the barrel, the plugs will look dark. Reading plugs is normally done after a good hard run at wide open throttle. They should be a nice light tan. Don't let it idle. Chop the throttle after the run and immediately read the plugs.

 

[R.Johnson]

Re: how to read a sparkplug?

jwilkey84: Where are you located in Iowa? I'm in LaPorte City.

 

[jwilkey84]

Re: how to read a sparkplug?

I am in creston

 

[Kymasabe]

Re: how to read a sparkplug?

I always thought that removing/replacing a hot plug was a bad thing to do. I was always taught in automotive to only replace plugs when the head was cool. Is it the same for marine engines?

 

[DJ]

Re: how to read a sparkplug?

I always thought that removing/replacing a hot plug was a bad thing to do. I was always taught in automotive to only replace plugs when the head was cool. Is it the same for marine engines?

Yup, same rule applies. Just DON"t LET IT IDLE. Idling blackens the plugs.

It doesn't take long for them to cool down since they're only running about 145 degrees.