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.

 

Sounds like its running very lean. I quick test would be to put the airbox lid back on (if you have it, or try duct tape if you don't) and see if that improves things. If it does, you need to install a larger main jet.

 

needs a valve job and has to be re jetted for your elevation. sorry no cheep way around it.

 

thanks. i'll try the tests and see what happens. i guess this is one of the challenges with buying used. you never know what you're getting into, nor do you have a history to work off of.
is the valve job needed because of the backfiring?

 

Both buyer and seller live in the same city and are at sea level. Why does he need to "rejet for his altitude?"

Chucktown: If it's popping on decel AND you smell exhaust fumes, the first thing I would do is check for an exhaust leak. Check where the head pipe joins the cylinder head and also where silencer clamps to the head pipe. Inspect the entire exhaust as well.