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Discussion Starter · #1 ·
Plug color only indicates how the jetting is for the RPM range you use the most. For most people that is high RPM which means it indicates whether the main jet is too rich or too lean. That is very important but having that correct does not mean your mid throttle jetting is right, or that your jetting off of idle is right, or that your idle jetting is right. I highly recommend a good jetting calculator (www.dragonfly75.com/moto/carbjetting.html) to know if the jetting is rich or lean between closed and fully open throttle.

Read the color on the first half ground electrode (not at its end) and end of the ceramic insulator. Gray or chocolate brown is the ideal color.

White colored ceramic insulators are caused by
1) too lean a fuel/air mixture
2) too hot a rating for the plug

Gray colored plugs can be caused by
1) use of aviation fuel
2) use of certain synthetic oils (Super M is one of them)

Dark plug color can be caused by
1) excessive amounts of oil added to the gas (mostly with group 1 and 2 oils otherwise known as mineral oils. Go to www.dragonfly75.com/moto/oil.html to see what group oils are in the most popular 2 stroke engine oils)
2) too rich jetting
3) spark plug misfiring possibly due to bad electrical connections or a failing ignition component (CDI, high voltage coil, stator coil)
4) too cold a heat rating for the plug

Be warned about lack of coloration at the side end of the center electrode that is more than a half millimeter. That is caused by too much spark advance or too hot a plug rating. Enough spark retardation allows all of the sides of the metal electrode except for the last half millimeter to have combustion deposits on it.

Cheap gasoline without enough deposit control additives can leave strange deposits on the plug. This is fairly common in 3rd world countries but also happens in 1st world countries. (see https://www.knowyourparts.com/technical-resources/engine/how-to-prevent-engine-deposits/ )
 

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Sparkplug reading is a dark art. The plug looks far too black for me. What you really want is part of the porcelain clear of soot. The plug must run hot enough to burn this off. My belief is along the line of where the porcelain goes from white to black is the checking point. Your plug is running too cold. If it is all white (basically soot free) then you are too hot. The other question is, did you kill the throttle at full noise, if not, coming to and idle and switching off is enough to soot the plug and hide the reading.

I've been developing an EGT and CHT display in my spare time and, I think this is really the only way to really get the jetting correct. It needs to be live as well and not data logging so you can see the temp change as you ride.

I once seized my race bike and in looking at the logger I had (after the fact) I could see the CHT rising each lap till it seized. If I had a real time display I would have caught it before it happened.
 

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Discussion Starter · #4 ·
Yes the plug is dark from slightly rich jetting. And its heat range is too cold because the soot is the same all along the ground electrode.
Which part of the ceramic do you want to be clear of soot?
 

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This is a photo from the web. The outside is quite sooty so may be from a 4 stroke. It probable should be a lighter grey but the middle is correct. In the old days with points and coil, when you had a plug like yours, the bike would often not start when cold. The rise time of the spark was slow and the high voltage would leak down the soot as the voltage rose and then not jump the gap.
35583
 

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Two years later I have since learned that the best way to jet a carb is not by plug color but setting it for the best engine power under high load and then looking under the piston crown to make sure the oil isn't burning black there which would mean the oils heat tolerance is not enough and you need a more synthetic oil.
www.dragonfly75.com/moto/plug.html
 

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Two years later I have since learned that the best way to jet a carb is not by plug color but setting it for the best engine power under high load and then looking under the piston crown to make sure the oil isn't burning black there which would mean the oils heat tolerance is not enough and you need a more synthetic oil.
www.dragonfly75.com/moto/plug.html
I can't agree. The piston looses it's heat through conduction with the cylinder wall. If you have a worn piston, it has to heat up more to expand to touch the cylinder walls. This extra heat burns the oil as the piston is burning hotter than it should be.

See here Top End rebuild

Synthetic oils may have a slightly higher temperature, but that would not be my goto solution for excessive heat. Find out why the piston is overheating first.
 

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Now it's me who can't agree. The piston heat mostly goes to the cylinder thru the rings and so excess clearance has little effect on piston heat. Synthetic oils are a must in high RPM high stress engines. Unfortunately most promotional literature for engine oils lies about how synthetic it is. I completely investigated this subject and to find out the percentages of different types base oil in each product go to my page www.dragonfly75.com/moto/oil.html
 
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