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Slight missing at 1/4 Throttle - '02 WR250f

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My bike just started doing this (I'm pretty sure I didn't just start noticing it). If I run at 1/4 throttle with a light load it misses a little bit. I've played with the idle screw, and that doesn't help. I've got a 42 pilot jet, and I'm still at 2.5 - 3 turns out. I've cleaned the pilot jet and main.

I'll probably try a plug, just because that's easy, then I'll check my valve clearance - although it's only been about 500 mi. Otherwise I'll try raising my needle - it's still in the stock position.

This bike has always run great for me, and it seems like this just started. I don't want to get the tuning out in left field.

Does anyone have any other suggestions?

Thanks,

Toby

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I've been fighting this problem for 3 months now. It has been a gradual increase since. At about 1/16-1/8 throttle I've got a lean misfire....very annoying sounding. I've increased the pilot from stock 40 to 42 and no diff.

I'm about to put a seal in the slider of the carb. Look at crf450 forum subject "hanging idle". similar problems.

the slider of card is in 2 pieces. Where the 2 pieces mate up there is a big circle making a male and female "joint" for lack of better term. There should be a cup shaped rubber gasket in there. I don't think mine had it when I had it out. I'm going to install a new one as my mechanic had a similar problem on a much 5 X's worse scale than I and that is how he solved the problem.

Be careful on reinstall as it's easy to get the plate on upside down. Plate's notch goes toward bottom of carb. Looks wrong but that's correct.

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I have had the same slight miss since my WR was new. I tried to rid the miss via jetting changes, and to no sucess. IMO, I am pretty positive it's an issue regarding the TPS sensor and it's sending some glitched info to the CDI. IMO, I feel its an ignition stumble. I remember riders swapping to an aftermarket ignition system, and the miss was eliminated. I don't notice the miss on my bike when heavily loading the engine MX'ing, but I do however notice the miss street riding holding the throttle at about 1/3 open.

Rich

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I had this same problem when I got my WR, and at the time, the board was full of conflicting opinions regarding if it was a CDI/TPS issue or a jetting issue. I upped my pilot jet and adjusted the pilot screw and the problem went away, and hasn't returned since.

It was a very aggravating "miss" that only occurred when I held the throttle steady at about 1/8-1/4 open, at constant speed.

Since the jetting solution is cheaper to try, I'd suggest you try that first. If that doesn't get it, try unplugging the TPS. I hear a lot of guys on 450's find that works for them, but I don't know if it adversely affects the performance of the bike. If all else fails then you could look at an aftermarked ignition system if you wanted. I'm trying to remember if it was called the X10, or something like that, but it had different settings for torque, top end, etc.

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OldnBold:

That sounds interesting. Are you talking about the big O-ring, essentially between the front and the back of the slider? Let me know if the fix works for you.

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OldnBold:

Are you talking about the big O-ring, essentially between the front and the back of the slider? .

yes. I had mine apart and didn't see one but at the time I wasn't looking (it may be there).

I'm waiting on the part.

I took barton's advice on this problem and tried a 42PJ and really put a air/chemical cleaning to the carb. Put it back together and really liked the performance of the 42PJ vs 40PJ but the annoying miss is still present.

I've checked the TPS and the readings it gives are all normal.

My mech had a prob for a very long time with a customer whose bike had a really bad hanging idle and no matter what PJ he installed or where the fuel screw was it wouldn't go away so the only answer was to have a really high idle just to keep it running. He'd installed my carb (b4 my current prob) and no problem. He spent hrs trying to fix this and finally gave up and diagnosed as a factory defect in carb. We discussed the crf450 thread where the guy siliconed the 2 pieces of the slider together and he tried it. Problem was much worse than before so he knew he'd changed something. he installed the gasket and problem completely disappeared. Keep in mind this had been a problem for over a year. It is a completely different bike to ride now that it doesn't have to idle around 4000rpm's!

I think that my slider is wearing and letting air past it. This front of the slider is called a float valve and is supposed to float and I'm assuming is mangaged by engine vacuum. Air getting past this gasket is air in front of the fuel circuits and can cause a lean condition.

I'll try and installed this week and ride this weekend.

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does the pilot air jet have an effect on that too? I beleive if you change the pilot jet, you're also supposed to change the pilot air jet. there is a chart in the faq about jetting the pilot air and fuel jets. :)

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oldnbold,

Interesting history. I've seen a lot of intake tracts leak and cause a hanging idle, but I'd never though of an internal air leak causing the same problem, but I can see how it would. I guess the key is that something is causing a lean condition, it's just a matter of figuring out where it's coming from.

does the pilot air jet have an effect on that too? I beleive if you change the pilot jet, you're also supposed to change the pilot air jet. there is a chart in the faq about jetting the pilot air and fuel jets

There is a chart somewhere about PJ vs PAJ changes. By following the chart, you get a consistent fuel/air ratio. I think the idea behind changing one or the other (PJ or PAJ) is to alter that ratio. So you could improve a lean condition by either using a larger PJ or a smaller PAJ. If you followed the chart, you'd raise or lower both jets, leaving the mixture about the same, and not helping the problem.

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I think that my slider is wearing and letting air past it. This front of the slider is called a float valve and is supposed to float and I'm assuming is mangaged by engine vacuum. Air getting past this gasket is air in front of the fuel circuits and can cause a lean condition.

I'll try and installed this week and ride this weekend.

Did installing the new gasket fix your problem? If it did fix the problem, please give more detail on which gasket this is.

I have a 2003 WR250F with a similar problem. The problem is a hesitation at 1/4 to 1/2 throttle. The engine acts like it is not getting enough fuel at mid range RPM. Once the engine speed increases, the engine seems to have a lot of power. The engine idles fine, and runs well at low speed when I am crawling between the trees.

I have checked the TPS, and it is OK. I have checked the coil, and it is OK.

Over the past 6 months, I have tried several jettiing changes. I have tried a new setting every two weeks. I have tried making the jetting richer by increasing the pilot jet. I have also tried moving the needle clip down. That did not fix the problem. I have tried making the jetting leaner by decreasing the pilot jet, and moving the needle clip up. None of the changes have solved the probem.

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Did installing the new gasket fix your problem? If it did fix the problem, please give more detail on which gasket this is.

I have a 2003 WR250F with a similar problem. The problem is a hesitation at 1/4 to 1/2 throttle. The engine acts like it is not getting enough fuel at mid range RPM. Once the engine speed increases, the engine seems to have a lot of power. The engine idles fine, and runs well at low speed when I am crawling between the trees.

I have checked the TPS, and it is OK. I have checked the coil, and it is OK.

Over the past 6 months, I have tried several jettiing changes. I have tried a new setting every two weeks. I have tried making the jetting richer by increasing the pilot jet. I have also tried moving the needle clip down. That did not fix the problem. I have tried making the jetting leaner by decreasing the pilot jet, and moving the needle clip up. None of the changes have solved the probem.

No, the gasket made not 1 bit of difference.

My problem sounds different than yours. Mine was a constant misfire at a throttle setting just cracking the throttle, maybe 1/16 -1/8. Under load, like deep sand the sound wasn't there. Hardpack or asphalt the sound was there. The motor felt fine but you heard it misfiring. Accelerate and the misfire sound disappeared with no bogging or performance loss. Chop the throttle and no backfire. A PJ change from 40-42 didn't help either.

I got off my ass and changed the JD needle from the blue needle 3rd to JD needle red 5th. I heard the problem 1 time, briefly, this past weekend with 100 miles of trail riding. Current settings @ 80deg, 600 feet ASL is 40PJ, red needle 5th, 180MJ (I'll lower soon to 178). Problem seems to be fixed (thank God as it was an irritating sound).

Your description of a hesitation sounds like the infamous bog. does this hesitation occur when you open the throttle or at a steady throttle? If it ocurs when you open the throttle(whack),you've got the bog and can be addressed by changing the leak jet in the bottm of the float bowl. A 40 LJ or a 55LJ are what most are using to solve this problem. Stock is 100 (yz's) so there is a big change in amounts of fuel sent via accelerator pump by changing this jet.

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