XR250 Stumble help please

[TexKDX]

Hello DRNers,

I have a '99 XR250 and am riding it in the mountains of CO. I think I have it jetted pretty well right now. Mods are: airbox screen and snorkel removed, baffle removed, pipe welds ground out.

Here's the problem: when lugging the motor on level ground and riding whoops, normally in 4th gear, the motor coughs badly when I hit the whoops. It will do the same thing in any gear if I am at lower RPMs and around 1/4 - 1/2 throttle and hit say rocks or roots. If I run a higher gear and scream the motor, then the stumble does not happen.

It sounds like float level in some ways, but this does not explain why the problem is not there at higher RPMs as best I know.

Any help would be appreciated. I'd like to be able to make better use of the torque available at lower R's. Thanks!

 

[SpeedyManiac]

My XR100R does this too one hard landings and hard, sudden hits. I don't think there is anyway to fix it, because it is just from the engine being jolted hard.

 

[penguin]

It sounds like a fuel starvation problem. 2 remedies come to mind. Install a TeeVent system to help equalize pressure in the carburetor float bowl, and check the float height. this is a common problem with many bikes. The fuel can actually start frothing in the float bowl and the engine tries to suck air bubbles. Higher float height and better venting help this.

 

[mtngoat]

Here's a link to ab XR4 fact sheet that discusses a similar problem and a float adjustment fix. May not be applicable to the XR250, but it sure sounds like it.http://john.rushworth.com/Motorcycle_Co_UK_archive/xr4faq/

 

[Danny8785]

If you are running a low grade fuel it could also be "spark knocking", just a thought

 

[TexKDX]

Thanks Mtngoat!! The way they describe the carb on the 400 is sounds just like the 250. I will try the float travel limit mod and see if it helps.

"Additionally the bike may feel as though it bogs over small bumps. To cure this you need to limit the down travel of the float from the standard 25mm to between 18 and 21mm. To do this take the carb off, remove the float bowl (paying attention to the position of the white nylon main jet baffle) and keeping the carb right way up, measure vertically from the edge of the carb body on the centreline of the main jet to the metal float arm. Carefully adjust the small metal tang by the float pivot pillars which the float hinge pin goes through. Bend this with a pair of small pliers till the desired measurement of 19.5mm is achieved. Invert the carb and check the float height (measuring in a similar manner) is 14.5mm, which is the standard setting."

Gotta love DRN!!!

 

[mtngoat]

Glad to help Tex. :)

Let us know if that's a fix. BTW, Honda sells a float bowl guage for around $25. Also, you might want to check to see if you have a dual-taper needle (since you'll have the carb out) and whether Honda supplies one for the XR250. It's a good thing.

 

[TexKDX]

OK, I set the float level per XRsonly as level with the carb body (lowered it about 1.5 mm) and reduced the float travel to 17mm. The job took about 10 minutes thanks to the easy carb removal on the XR250. While I was at it I went from a 125 back up to a 128 main as I suspected I was getting too lean trying to get the stumble problem solved.

Off to the same exact whooped-out 4th gear trail... guess what - PROBLEM SOLVED!!! No more stumble! I can torque the thing around in 4th no problem. Going back to the 128n improved my top end power too. I may even go up one more on the pilot as the fuel screw is 3 turns out now to get over the closed throttle backfiring.

Thanks for the tip, goat!!!:cool: :cool: :cool: :cool: :cool: