Help!! I just picked up a 99 klx300 and it backfires constantly, has nasty exhaust fumes, and is very sluggish low end and top end. What's going on???
It came w/ a K & N, no airbox lid, DynoJet stage 2, big gun exhaust w/ the spark arrestor in place. I live in Charleston, SC at sea level, and the bike came from Charlotte, NC above sea level. Could the elevation difference be the problem? the previous owner said the bike always ran great for him, acted surprised about the backfiring, but did note that he barely saw a difference in performance after having the Dyno kit installed. Need to go riding now! Any help would be greatly appreciated.

I brushed off the K & N, cleaned the plug, changed the gas, adjusted the needle (Dyno says open 2.5 turns) i did everything from closed to 3.5 turns, no difference.

 

I had a very similar issue with my new-to-me 2001 KLX331. It was running rich no matter what I did. After pulling the carb and changing jetting about 20 times - I had to admit it wasn't a jetting issue.... although the bike definitely perked up with the dynojet kit. I had cleaned the carb - but not well enough. My airinletsto the pilot and main jets were completely plugged. So, I was rich from a lack of air rather than too much fuel. I cleaned them by removing the main and pilot jets, filled the holes with carb cleaner and blew the snot out of them with 40 psi of compressed air. Now I actually get air into the jets and it runs much better. Good luck.

Dean