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04 KX250F Dog when cold

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Hi Everyone,

I have recently purchased a used 04 KX250F. It is an absolute dog when cold,meaning when I start it with full choke - it will start ok, but if you turn the choke off too soon it splutters and dies...horribly...however once it is warm it idles ok...and runs very, very well on full throttle....

The other issue that may be related is that it backfires on de-acceleration. As this is not a new bike, I know the previous owner had pro-circuit pipe on the bike and replaced it with the stock pipe when I purchased it.

Does anyone know what might be causing these problems?

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Well Im not sure about the starting isue, But it is noraml for a cold motor to die when you turn the choke off if the bike is not warm.

But for the poping on decel, well that means that its eather too lean or you have an exhaust leak somwhere, I would start by turning the fuel screw out a half a turn, and keep going out untill it stops poping, But if you go out more than 2 1/2 turns go with the next size bigger pilot jet and start the fuel screw back at a turn and a half out, and go from there.

The rule of thumb is.

More than 2 1/2 - 3 turns out, go to the next size bigger pilot.

Less than 1 turn out, go with the next size smaller pilot jet .

:cry:

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Hi Everyone,

I have recently purchased a used 04 KX250F. It is an absolute dog when cold,meaning when I start it with full choke - it will start ok, but if you turn the choke off too soon it splutters and dies...horribly...however once it is warm it idles ok...and runs very, very well on full throttle....

The other issue that may be related is that it backfires on de-acceleration. As this is not a new bike, I know the previous owner had pro-circuit pipe on the bike and replaced it with the stock pipe when I purchased it.

Does anyone know what might be causing these problems?

First I would check the jetting in the bike. If he had a different pipe on it the jetting has probably been changed and I think Pro circuit recommend some pretty lean jets. Stock jetting is 178 main and a 40 pilot. I'm running a KX250f with 175 main and 42 pilot and a RMZ250 with 178 main 40 pilot. Both bikes are different and the KX seems to have more snap down low.

Like the other posts say check that everything is tight and where it should be. My bike used to pop and backfire when it was cold but the richer pilot and warmer weather cured that. The richer pilot will probably help with the starting as well.

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The cold weather thing sounds normal.

and are you sure it's backfiring or just cackling? My 250f and both of my friend's 250f's cackle on decel. We both have the jetting adjusted fine, unless it's horribly loud, it's a normal 4 stroke sound.

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