WR250f 02' Tuning Question

[Duwvesmith]

I am trying to improve midrange power on my WR250f.

The problem: The engine stumbles and coughs.
When: At about 3000-4000 rpm when riding at a steady pace, roll the throttle back to wide open. The engine then stumbles and coughs and has very little power.

I have tried several things to improve this but have not found a solution yet.
I am running race gas.
I have cut the gray wire.
I have removed the throttle stop, the airbox snorkle and the exhaust insert.
I have raised the needle 1 notch (lowered the clip)

The above mods have made great improvements in low and high end power. One other thing I have tried is changing the timing of the exhaust cam back 1 tooth so the timing dots are level with the head (YZ spec). This improved the stumble but I was told to change it back by Thumper racing. Apparently the WR ignition is timed to match the original cam orientation. I did this and now the bike runs like **** again.

Should I go ahead and change the cam to YZ spec again, or is there another problem I have missed?

I need someone who has solved this problem to give me a hand. I would greatly appriciate it!

Matt

 

[Boozer]

okay. i like to think i have the WR250F pretty much sorted out.
looking at your specs, i am guessing you are running too rich, especially on the needle. i would first try dropping the needle 2 clips from your current position. this may make it crisper in throttle response too.
when you say it cruises fine until the gas is chopped, then it's time to look at your acc. pump duration/timing. thumpertalk.com has plenty of information on this issue. i would suggest you remove the air box boot from the carby, and wedge it back behind the frame. look down the throat and you will see a small brass jet sticking up. this is your acc. pump squirter. keep an eye on it while cracking the throttle all the way open, and hold it (engine off). if you have a squirt that is about 3 to 4 seconds long, then you need to do the BK mod. i'm guessing your squirt is too short, and i too had a short squirt problem. if it is the case (squirts for a fraction of a second), then you need to do the HB mod, which also involves doing the BK mod.
i have done the HB mod, and once set up, my bike runs great. i can crack the throttle open at any stage, and the engine will not hesitate. before, the engine would shut down.
i too have moved to YZ timing, and initially the problem became worse, but like i said it is fixed now. i would not go back to WR timing, as you don't need to. the only mod i would consider when going to YZ timing is the air cut valve mod, which is easy to do. i have been told you cannot do the mod while running WR timing, so it's up to you. oh, when you say back one tooth, i'm guessing you mean clockwise, right?

 

[Duwvesmith]

Thanks Boozer!

Hello Boozer!
Thanks for the information on the mods. I assume that Thumpertalk sells these HB and BK mods. I'll check my carb to see what the acc pump is doing.

To answer your question, yes I rotated the cam clockwize.

Thanks again, I appriciate you help!

Matt