Maintenance & Technical > KX250 / KX125

high steady rev at start up.

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Wizzy214:
99 kx250 just working out all the bugs.
every time i try and start the kx it revs up super high super fast. when i twist the throttle it bogs out and dies. and this is with the choke on, but when i turn the choke  off its at such a high rpm it doesnt do anything. and the kill switch doesnt do much when pushing it as hard as you can.
what could be causing this?
thanks

Jeeks:
Sounds like it's starving for fuel.  First off, you should really fix that kill switch.  Check all the passages and jets in the carb, and look for an air leak.

Hillclimb#42:
Revving up is a lean condition. So it is not getting enough gas or is getting too much air at idle. Then the bog is off the idle jet so indicates plugged main jet or busted reeds. Sounds like you have two things going on at once. Pull fuel line from carb, and check gas flow from petcock. Check for air leaks around head or base gaskets. (would look clean or wet in seam), pull carb check jets by looking thru them. Verify they are the right ones for your temp and altitude from jetting chart. Pull reeds and visually inspect for cracked or missing petals. Since you would already have carb apart check float adjustment. Hopefully you have not had that hapen much, because when it leans out and revs high like that, it usually is melting the piston. Actually that kill switch may be working alright. When that cylinder leans out it gets hot enough to run without spark. Make sure idle screw is not all the way in and slide closes all of the way. There should be a little slack in throttle tube that you can feel in the grip.

don46:
lean condition, plugged jets, wrong jets, air leak, betting its one of these, what size is the main and pilot?

Jeeks, you have a 99-02 250 what jets are you running? I would guess about a 160 main and probably a 58 pilot.

Jeeks:
I'm not fussy.  I stick with the factory jetting, except one bike that has a slightly rich pilot.  Don't really need the choke to start it.

I think, like others said, something is wrong otherwise it would have run lean before.  You really need to retrace your steps, starting from the cylinder back to the air filter.

One thing I have found with my 2000's is, on cold starts the bike rev's high at first, so to reduce any damage to the engine I throttle the choke lever to keep the rev's at a respectable level while she warms up.  It only occurs in the first 20-30 seconds, but it's not good practice to wind up a cold engine.

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