What is a good color for spark plugs?

6 replies to this topic
  • Thumpin_426

Posted September 15, 2001 - 02:38 PM


I just finished the MK mod, which is unbeleivable. My plug went from being a sooty black to a Very light tan, almost white. Is this normal?

  • yzf

Posted September 16, 2001 - 03:18 PM


black is rich white is lean tan is good

  • SUnruh

Posted September 16, 2001 - 03:25 PM


light tan is what you want.
HOWEVER, do NOT look at the tip. that does not count. you have to look WAY DOWN INSIDE where the insulator meets the steel. the color of the tip means nothing unless there are silver (aluminum) balls on it at which point you get to install a new piston from being WAY WAY too lean.
to get the correct jetting, you need to take a dremel with a cutoff disc or a hack saw and cut the threads off the plug. a while back i posted a picture in the 250F pic forum that i had done this to.

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  • jaybird67k

Posted September 16, 2001 - 04:17 PM


Dont forget that the type of fuel you run has a direct influence on the color of the plug, and according to Russel at Thumper Racing there is no way you can lean this bike to the point of being harmful to the engine, because if you get to lean it simply wont run.
Whitish tan is a good color for pump fuel, Whitish grey is a good color for leaded race fuels, and white is good for unleaded race fuels.
Dont forget that if you run leaded race fuels as I do you need to change your plug about once a month because of the lead deposits that build up under your grounding arm on your plug. These lead deposits make your bike cut out under heavy load at full throttle operation, and it is also important to note that your compression will rise after running leaded fuel because of the lead deposits that build up in the dome of the combustion area.
Leaded fuels require a little more maintenance but in my opinion are better in the long haul.
Sorry to go on and on but I was bored...........


  • G-Man

Posted September 16, 2001 - 04:45 PM


Thanks Jason, some good info.
Hope it's all true!!!

Moto to Live
Live to Moto



  • Thumpin_426

Posted September 17, 2001 - 08:45 AM


SUnrah, What do you mean by, "To get the proper jetting you need to cut the threads off the plug"? I'm running 50/50, 93 Amaco and 110 Race fuel. My plug is nearly white with just a tint of tan at the tip.

Let me know what you think..


  • Boit

Posted September 17, 2001 - 01:29 PM


Jason: I agree completely. C-12 leaves a light ashy gray tint to my plug and exhaust system. I've found the 426 to(at least mine) be somewhat sensitive to jetting. Just one size in change on the main jet makes quite a difference. The pilot circuit is pretty easy to dial in once the proper pilot jet is used. Small incremental adjustments with the pilot fuel screw is all that is required. I like my pilot circuit tuned to give a slight gurgling....or muffled popping on decel. This gives me the best throttle response if I need to really whack the throttle open quickly at low revs. I have no bogs, blips, hesitations, or other glitches.

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