What is likely cause of misfiring on new bike ?


I've got a new KLX300, just past the breakin period and it started misfiring under acceleration. Nothing really seemed to preceed the problem, no stalls, drops, etc. The gas in the tank is on the low side - I originally thought it was running out and switched to reserve but the problem still persisted. I'm going to fill the tank & try again but I have to trailer the bike to the riding place an hour away and want to find some solutions before I go. Is this most likely a fuel problem ? Breakin prolem ? Emissions ? Spark ? I'm unfamiliar with diagnosing engine running conditions...... Thanks for any input !


bad jetting.


Your KLX is jetted way lean to meet California's standards. Go to the KLX group and search around for just the same problem and you should be able to find your solution. If that does not do it, go to the jetting group.

The popping you describe happens under deceleration and your RPM's are up there right? It happened on my bike till I got the jetting squared away. :)


OOPS!! I just re-read your post and you said it happens under acceleration, sorry. :D Check with the groups I mentioned for your cure.

It seems like the answers are leaning towards jetting - the bike ran fine for the first couple hours - why would it suddenly change if jetting is to blame ?

Good question-to me if it's jetting then the plug should be a good indicator.Different temps will change the mix.Check the plug for color.You might drain the fuel bowl and check for any debris or water.Is your air filter clean?

Is this is a new bike and under warranty. If it is take it back to a dealer. This is not your problem it is theirs.


If its that new, take it back to the dealer. If he won;t touch it you have some options:

1) Install a dynojet kit- works real well. Also, the CA models cover the air screw under the carb with epoxy. You can break it out and adjust it for deacceleration popping.

2) Check the valve clearances to make sure they are in spec.

3) My friend had a KLX that drove him nuts. He traded all kinds of parts, carbs, black boxes. Someone finally told him to check the brass screw/nut in the spark plug cap to make sure it was tight. That was it. It is a retainer for the resistor placed in the cap, and it was loose causing a sporadic misfire.

Look at the sparkplug first....It can tell you alot if you know what to look for.......Black and shiney center electroid...means its getting oiled from somewhere Aircleaner oil?

Black and sooty, too rich. Choke is on or you ran it too long on the choke and

loaded the plug up (fouled)

White with little specks of metal...

very lean ...air leak at maifold?

Try a new plug before you do anything else....then get worried..Probably a plug with one of these problems...

Plug cap being loose like Burnrider said

that makes sense also...Good luck.

BC :)

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