Hey guys, hope ya dont mind me posting this here. Just picked up an 07 KLX300R. Bone stock AFAIK. It fires up first kick every time cold or hot. Idles nice and smooth. Runs great with the engine under load while accelerating and higher rpms, but pops when decelerating or at mid range rpms and not under load. Also stumbles at mid range rpms and not under load.
I plan to get the valve clearances checked. No idea how many miles are on the bike. The odometer is.only a "trip meter" that can be reset. From what i have read on here so far the valve clearances need to be kept on top of, but i dont think that is my issue here. Im leaning towards the little felt seal at the throttle plate rod leaking just a little bit. I have not had a chance to check this yet, but i will tonight with a can of starting fluid.
I just wanted to get a thread started and some ideas rolling in. What say you?

 

If you refer to Mikuni, you will find they say a bit of popping under deceleration is not a problem, closed throttle creates a lean condition. Some people fatten up the idle mix or a bit bigger pilot jet, because the popping bothers them. I don't, doesn't bother me. You'll hear properly jetted race bikes crackling as they decelerate. Don't confuse it with serious "backfiring" through the exhaust, that's a different story.

Now if you have stumbling that'sanother issue. Try this - open the "choke" which is actually an enrichener circuit. If the stumble gets worse it may be too rich, if it gets better the carb is a bit lean. Change the needle clip in a higher slot, dropping the needle, if too rich, lower one slot, raising the needle, if too lean.

Some of us use the 300 needle because of that ability to adjust it..Seems some of the early KLX250 riders experimented with the 300 jetting and it worked out well..

 

The mid range stumble can be caused by a lean carb. Is the lid of the air box still on the bike and has the exhaust been modified in any way? Both of those things can cause the lean condition IF the needle in the carb is still the non- adjustable one. Check the needle to see if it has an adjustable clip on it.
Lack of valve clearance usually show up with hard starting.

Ride on
Brewster

 

I sprayed starting fluid on the throttle return spring with the bike at 1/4 throttle and the engine smoothed out and revved higher. Im leaning more and more towards the felt seal on the butterfly shaft being worn as posted here: Butterfly shaft seal replacement