I have a 400ex. I added a Duncan Racing pipe and a K&N. The dealer adjusted the carb and told me the original jetting was fine. It started running rough so I did a spark plug read (thanks hot_shoe_cv). It was on an old plug and it was white/gray. So I took out the stock 148 and put in a 160(this is what Duncan racing recomended for Oklahoma). I put in a new plug, warmed it up, and opened up the throttle for about 3/4 mile. Pulled the plug and it was as white as snow. Do you think I still need to go higher? I see where some people are using 165s and 170s. What does anyone recommend for Oklahoma riding? Thanks

 

it sounds like you have an air leak. Check your carb boots to make sure they are fully on all around after your rejetting, and that the clamps are tight. Even if the 160 jet was a little lean your plug should not be white; some where you're getting serious air around the carb, rather than thru it.

 

Put your airbox lid back on and re-check. If it turns the correct color, I would recomend a 165 or 170.

Matrix

 

I checked all of the boots on the carb. I pulled them up tight and tightened them down. I then put in a 168 main jet and followed the above procedure on a new plug. Around the threads is black, however the metal tip over the electrode is still white/gray with a tint of brown. With all three jets I have tried (148 stock,160,168) I have had backfiring (sounds like from the pipe) after releasing the throttle when it was fully opened. I am running a K&N with the air box on and no holes. I am not sure what too do at this point. Any suggestions would be appreciated.

 

Considering all you have done & still little or no results, the only thing I can think of to do would be to go back to square one & replace all the stock components & then check for proper jetting. If it is neither lean nor rich the replace each aftermarket item & test as you go till you locate the problem.

 

Don't just check the tip of the spark plug. You need to check it down inside and take a look at the smoke ring that forms at the base. I still think you are running a tad lean. I'd go up by two's until your running rich and then back off. Your sputtering when you let off the gas is caused by the pilot jet not being fat enough or the air screw not being set right. Good luck