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Accelerator Pump O-ring Mod

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I just bought some leak jets from JD Jetting.

The kit also came with a couple of O-rings. The O-ring goes around the pump timing screw and pump linkage to increase the pump pressure when the throttle is opened.

I first tried just a smaller leak jet and it seemed to improve the throttle response. I only had a short time to test ride and then quickly installed the thinner of the two O-rings. I'm not sure it improved the performance, but I only had a short time to try it.

Does a WR250F with a de-restricted airbox and aftermarket muffler/header typically need the O-ring mod to increase the pump pressure? I still plan on doing some more test riding this weekend.:banana:

Edited by Sofiedog
fix typo

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Yes. The work on the AP has little to do with bike modifications and more to to do with how/where the bike is ridden. After you have theoring on, you may need to fine tune the AP by adjusting the AP tming screw. Keep good notes. Turn in 1/2 turn increments clockwise. Test ride after each tweak, do not just rev the bike.

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I found the "O" ring idea to be a subtle improvement (but noticable) to throttle response. Fuel economy has not changed. This also worked on my Husky 450. It is a worthwile exercise, particularly if you ride in the tight, ie you come round a tight bend and a log is across the track. The throttle response inprovement gets the front wheel up a tad quicker.

Note, be careful with you right wrist on steep descents, the improved response will make small throttle adjusments more impactive going down hill.

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Thanks for the input. Sounds like you guys believe it's worthwhile to put time in to tune the AP circuit. It probably will take a while to fine tune the leak jet size and timing screw. It certainly was worthwhile to spend the time tuning the main jet and needle clip position. :banana:

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Took the bike out for a trail ride for a couple of hours today. Rode most of the time with the thin o-ring. I then removed it, rode for a while, and then put it back on. Based upon my seat-of-the-pants test, the thin o-ring felt like it had a positive impact on the crispness of the throttle response across most of the RPM band.

Even though the bike is running well, I enjoy tweaking on engines.

I'm not sure what I'll try next. I may try the thicker O-ring with the existing leak jet (#40). Then, I may try different leak jets with the preferred O-ring. Then, I may try different AP timing screw positions with the preferred leak jet. Make sense?:banana:

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Its just plain fun messing about with the beasts isn't it. keeps you out of the Missus' way.

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