My brother's 400ex has a WB E-Series pipe and silencer, WB rev box, 6 holes drilled in airbox lid, K&N filter and outerwear. Everything else is stock including the jetting. He is running NGK DPR8Z plugs. The problem is after about 4-6 hours of riding the plug color is solid black (dry) as far down the insulator as I can see and the quad misses and backfires until the plug is changed, then it runs perfectly again. Is never smokes at all, although between the discs on the silencer there seems to be some black carbon build up.

My question is what is causing the plug to foul out like this so soon. Is it the jetting? I know if my Banshee plugs were this color it would be running rich. Any help would be appreciated.

Thanks guys...

00Banshee

 

Jetted to rich I think. Preform a Test and find out.

 

What year is his machine? Usually these mods would require/benefit from rejetting. I agree that it sounds rich by the plug, but 4-strokes can be a little more challenging to jet by the plug. Have you done a "plug chop" to check the jetting? If he doesn't ride at WOT all the time, and you just check the plug whenever, the usual process may not tell you all you need to know.

 

My guess is that since he did all those mods his engine is running VERY LEAN. Reason being when you make the engine breathe better by doing mods to the intake and exhaust you need to increase the fuel since you increased the air flow. When an engine has an excessive build up of carbon as you describe it means that you are BURNING OIL. Why are you burning oil? You might ask. Well Since your engine was running extremely lean (and hot) you probly melted your oil rings and now you have blow by from the crank into the top end. This would explain the excessive carbon and the missing and sputtering. How often does he have to add oil? (if he does add oil) You can still burn oil and not have smoke at the same time. The engine could be running so hot that it in fact does burn all the oil. 2 strokes can burn oil and smoke very little and so can a 4 stroke.

To sum it all up I think your brother needs a new top end and needs to rejet his carb. Sometimes in life lessions like this can be expensive.