Spark Plug Tell Tales

[Jason2tpa]

Ive read that plugs can tell you a lot about what's occurring in the cylinder. But I haven't found out what it means when 1/4 to 1/2 of the plug surface is clean (unvarnished) and the remaining is brown (varnished). And also if it means anything as to which side of the plug is varnished/unvarnished. Your thoughts? Thanks!

[robert graham]

My guess is the direction the fuel/air mixture is entering the combustion chamber relative to the plug's position in the combustion chamber.....My C90 3 cylinder 2 stroke Yamaha plugs are generally not perfectly uniform in their carbon/varnish deposits, with maybe a bit more on one side of each plug....guess it's normal?....I sandblast my plugs anyway between uses, so they're generally nice and white, but after a few hours running that tan/oily/carbon/whatever is always there. That little Harbor Freight plug cleaner has paid for itself over and over....

[TownsendsFJR1300]

Agreed with ^^^.

The more time you have on the plugs/motor, the more even the color will be...

Fuels coming in from the intake (of course), burning, then exiting the exhaust valve(s).

To get an even color around the outside is difficult. **Your really ONLY paying attention to the electrode/ground for your main inspection.

Carbon build up on the outside (near the threads) will just show pretty much the plugs age, need replacement, etc. If oily, obviously some potential oil issues (rings, etc). But for general operation white or black are the extremes, tannish, clean, is where you want to be..

[SeaDawg3]

RG is correct,

it is easy for people to be mislead as to a plugs color. and mostly the incorrect way to read a plug.

in most cases a run at wot is necc then shut down the engine and read the plug. Looking through a spark plug viewer or magnifying glass down into the core of the plug is where you will see the correct indication. The plug has to reach peak temps and wot is where it will have the most fuel. If the AFR`s are correct you`ll get good color.

you will often see a cleaner area on the core nose bc the burn path is shrouded by the ground electrode. meaning after the fuel is ignited is it essential going around the leg of the electrode.

an indication of good plug temps also comes from the dark/oily threads from the ends and the core nose for any signs of blistering, and not to be confused with fuel additives that were not burned off.

also and most likely on the 2 strokes you find a clean area on the plug base where the threads end, as mention this location is from where the plug stops when tightened and in relation to spark kernel and flame propagation. There are transfer ports in a 2 stroke, evident on the piston crowns, known as piston wash.
In the 4 stroke world, a lot of speed guys index the plug.
I have not experienced an significant differences doing so in the 2 strokes.

there is a whole lot to understand when it comes to reading plugs and most just think it`s just the color closest to the end of the electrode.

several factors attribute the color.

afr`s, lean, rich, oil, low compression, oil ratio, 2 stroke oil`s especially the synthetics will make the plug look lighter bc it burns cleaner. also some very refined mineral oils will do the same.

a well performing engine with normal compression and correctly adjusted carb
will yield equal color in all cylinders. otherwise that would be an indication of something wrong or a known reason if you`ve changed something.

I had an efi 2 stroke that you`s swear was lean, bc the plugs were white for a long time... so you had to put some time on it for the plugs to show any color. even with a fuel con*****er.

[Jason2tpa]

Ok guys thanks all great info.