1998 KX250 bogging in at high RPM

[bigspeights]

My 1998kx250 has a problem. at high rpm the power seems to come on and off and if I hold the power on it boggs down a bit and i lose some power. it runs great in the slow tight stuff but crap on the open straights. I have set my float so that it is as high as it can be with out overflowing (I don't know any other way to set it), I have cleaned out my carby, I have cheaked my plug and it is light gray/brown colour, topend is only about 10 rides old, bike starts first or second kick when worm. I have a Bills pipe on it and I am not shure about the jetting but I have never had problems before. Could it be the solenoid on the carby not working and if so how could I test this.

I really want to go riding this weekend and need to fix the bike to beat my buddy's that Ride KTM 450's

[hughes]

So this issue came on all at once? Do you have any reason to suspect crank seals? Good fuel supply from tank to carb? Theres a filter screen on top of the fuel petcock inside the tank. Maybe some else can give you more info about the 98 kx250 carb.

[KXcam22]

Things to check,

1. Gas cap venting: will restict fuel flow at WOT
2. Jetting: How does your plug look at WOT.
3. Power valve solenoid:  To test this uplug the electrical connector to disable this.  It there is no change in how it runs then it is NOT this.

Set your float level back to the stock level.

Give these a check and post back. Cam.

[azdirtrider75]

I had a similar problem a few months ago with my 2002 KX250. It turned out that my gas cap vent hose was kinked and caused a vacuum, reducing the fuel flow to the carb. At slow speeds it ran fine, but at mid to WOT it would bog.

Good luck

[bigspeights]

Thanx for your help so far. There is heaps of fuel coming from the tank to the carb. how do I set the float to factory setting. The problem started on my last ride. Just before the ride I had changed my float level because fuel was coming out the over flow. I think I have changed the float back now but I don't know how to set the float properly. does the soionied in the carby come on at high rpm to reduce the fuel flow. if so if this was not working would that mean that if I unpluged it and the bike ran the same this would be the problem. could it be that my bike is getting to much fuel at high rpm. also is there an easy was to make the seal where the pipe goes into the head better, some oil splatters out of mine.

[hughes]

If the peoblem started right after you changed the float height then maybe you have got it out of proper adjustment. Do you have the service manual for you bike? In most cases you will invert or hold the carb at 90 degrees, so the float is facing upward and you measure the height from the top of the float to the base of the float bowl gasket area. Make sure the needle is seated but the plunger is not seated inside the needle. Some floats have a seam in the center of the float and you can just line it up with the gasket surface for the float bowl. The service manual will provide more info if you got it. Flaot height adjustments is just something that you get the hang of after many carb tunings.

[Jopiz]

Definitely the fuel level.
Here is a pretty good write up about adjusting fuel level. http://www.allthingsmoto.com/forums/f-15/how-set-your-motorcycle-carburetor-float-level-more-commentary-13608/

[hughes]

Yes, good write up and should help  bigspeights with his carb. good fine.

[bigspeights]

I have set my float so that it will close the fuel line coming from the tank when the fuel reaches the top of the fuel bowl (within 1mm). the bike is still not running right and still has power loss at high rpm. I don't know what to do. fuel seems to come out of the tank with out any problems and the bike still starts fine when worm.

[KXcam22]

I would suggest unplugging the power valve to see what change it makes. The power valve opens at WOT to add more fuel and richen the mixture.  If it is not working you should see the bog stay constant at WOT and not change at all. Cam.

[DoldGuy]

Not to speak for Cam, but he might have meant the Power Jet (on the Carb).

[KXcam22]

Thank you!  what was I thinking.  Definitely the wrong terminology.  Yes I meant the power jet on the carb. Cam.

[hughes]

I didn't write this "how to" but make sure yours is assembled correclty and set properly. click on photo and then you can enlarge it.

[Jopiz]

That's the exact one i posted earlier. Its found on ATM forums too.

[hughes]

Yes that's true. I saved it as a picture/photo that way we always got it around. Some times links to other sites seem to fail or go away.