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02 yz426 wont rev

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I just got an 02 yz426. It seems to be well cared for and starts fairly easy. It idles good but it wont rev out in any gear. Gets about mid range and gives up, backfires a little on accel and decel but no serious popping. The bike seems like it has good compression but I was told it needs rings. Seems more like a small main jet or running out of gas but the plug is black. It has an after market full exhaust but no markings to tell whos it is. Any help would be appreciated. Thank you.

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The first thing to check would be the air filter, especially if it seems to run up better at part throttle than at full. Another possibility that doesn't come up often is either a main jet that has fallen out. If the bike seems too quiet, or sounds choked up, you could have a collapsed perforated core tube in the silencer.

The rings are not implicated in any way, nor are the valves. Lack of compression affects low speed performance more than high.

Does it miss, or just run flat?

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I bought a used WR400 a couple years ago that did the same thing. Ran great on the bottom, idled perfectly, but had no top end power. I put a 170 main jet in and the problem went away. It turned the bike into a rocket!

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Thanks for the replys. It doesn't seem to miss, just runs flat. The filter is definately in bad shape. The main jet didn't fall out but it only has a 162. I was guessing a bigger jet also. It's definately not quiet, louder than heck as a matter of fact. Who makes a good quiet pipe for this machine that doesn't compromise power.

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