ATV Florida Forum

OK i have a kymco MXU 250, and have had it for 2 years now and this is the first problem It has had. It does not want to stay on idle, if I rev it, it runs and starts perfectly(except for more backfire) but if I let go of the gas it just turns off. Same thing happens on choke. I alway run it with 93 octane. And the idle screw is fine.
I dont know what to do. Please help.

Sounds like a lean condition, if you're backfiring and stalling out.


Pull your spark plug. Is it dark and wet?

Maybe you need to adjust your throttle cable? dunno if that would make the backfire though..

Pilot jet might be cloked up. I would pull the carb off and clean it real good. Make sure you can see light through all the jets.

Did you forget about us?  If it's both backfiring and stalling, your most likely culprit is a lean condition do to a gunked up pilot circuit.

Like Holeshot said, give your carb a good going over.

Pay attention particularly to the pilot jet.

another possability is a burnt valve or cracked valve ,clean carb thourougly if the same thing happens do compression check and leak down test.

just pull the bowl off and cleen the main and pilot jet and u should b fine.

mine was doing the same thing, took it to the dealer under warranty and just turned out to be condensation in the carb float bowl

Ok thanks. The bowl is the bottom part of the carb rite?? and can there be gas in the engine while i do that??

make sure you turn the fuel valve to off on the bike ,most carbs also have a bowl drain .if so drain the bowl into an approved container (bottom of a milk jug works too) then remove carb from bike and clean .P.S. I would'nt smoke or play with matches while ya do this

Ok thanks.

ok, this is what i did to it, and everything looked clean and fine. is there something else i should have done??


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Did you pull the jets out and make sure there clean? The plastic piece wans't laying there when you pulled the float bowl off was it? (just making sure)

yea the platic piece was just layin there. And were are the jets?

the jets are above the bowl .They are in the half of the carb that is still attached to the bike normally they are either hex shaped or have a screw driver slot in them .Remove carefully .the threads are very fine and can be cross threaded or stripped easily.That plastic piece normally goes around the main jet .Unless it broke I dont know how it would just fall off .But then again I never pulled that exact carb type apart before ,but most carbs are simular.

O ok thanks.. Ill work on it tomorow.

yeah i had the same type of problem on another 4 stroke motor. pull the carb completly apart and clean it. even if it looks clean doesnt mean it is. use a paper clip or fine guitar wire to get into all of the small holes in the pilot jet. pay atention to how everthing comes apart. and make sure you put it back together the same way. make sure the float is set right, ussally about half way or so. if gaskets look torn or old replace them. if you cant get the  gasket parts then go to the autoparts stor and get raw gasket material and make copies of your own, its easy and works fine. dont overtighten the screws you will only possibly strip them. hope this helps. good luck!!!!

ok, that looks more complicted but ill try.

Don't worry you'll be fine. Just make sure and do what yfz450runner said and pay attention to how things go before you take any thing off. I personally like to draw a small diagram on how things go. It also helps to take pictures and to have a very clean area to work with. Get yourself a clean bright cloth and layit on a nice flat level area and put all the screws and parts you take off in the same order you took them off. Also make sure you really clean your bike before starting any work on it.

Good Luck!