02 YZ426 questions

8 replies to this topic
  • rufusz

Posted August 13, 2008 - 06:02 AM


I changed the exhaust to a quieter one for SM riding (don't know what brand/make, got it with the bike), also the header pipe, and noticed that there is no gasket, or it's a lot thinner than the gasket on the original one between the header pipe and the rest of the exhaust (slip-on muffler on pipe). Can this cause excessive popping? It's not like popping 2-3 times louder on decel, it's popping continuously like machine-gun fire :) Or mixture problem? (Put Hot-Cams exhaust cam, but didn't adjust anything in carb). What can I use as a replacement gasket? Is it something Yami specific, or can I get it from automotive stores also?

  • matt4x4

Posted August 13, 2008 - 06:42 AM


the yamaha washer is a copper crush washer - likely, yours has had it - before playing with your jetting (which could also be a problem) I would go get a new washer first - at least you will know it's sealed for sure before hunting other areas..

  • kz2zx

Posted August 13, 2008 - 07:40 AM


I think I read someplace on here that Grayracer had recommended an 8ga copper wire ring, and the guy who did it posted up a sincere 'thank you'.

Maybe here: http://www.thumperta...884#post6731884 ?

  • grayracer513

Posted August 13, 2008 - 07:47 AM


It's worth checking the header/head gasket, but the gasket he's talking about is at the joint of the header to the mid pipe. Most after market exhausts do not have a gasket here, and some can leak a significant amount of air when they get old and loose. This could be the cause of your decel backfire, and should be checked before trying to correct it with jetting/adjustments.

If it does leak, try some high temp silicon sealer. Let it cure for 24 hours before running the engine.

Bear in mind that some decel popping is a sign of a well calibrated YZ426/450, so don't fight to eliminate the last little bit of it to the point that you have your idle richer than it should be.

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  • rufusz

Posted August 13, 2008 - 09:23 AM


Yes, grayracer, I'm talking about that gasket. Will try the high temp silicone, but would like it better to have some "real" gasket there, because I know that silicone is a kind of rubber, but it's not meant to support vibrations and can get loose of the surfaces, and then it will seal as a simple "O" ring.
Maybe the stuff that is inside the silencer, don't know what's the name...wool, or what the ?@%! is it's name :)
BTW how to check if it's leaking? Start up the engine and plug in the exhaust?

  • kz2zx

Posted August 13, 2008 - 09:57 AM


No doubt that I misread it with the best of intentions :)

  • grayracer513

Posted August 13, 2008 - 09:57 AM


That's one way, but it doesn't tell you whether it is drawing in air during operation.

The pipe was made to provide an adequate seal with no gasket at all. Running a film of silicone is a valid method of sealing older ones that loosen and leak. Us a high temp type, black seems to work best, for some reason, and let it cure as I said. If the leak is at that point, and causing your problem, this will immediately affect it.

As the engine runs, the silicone will actually "burn" to a harder remnant of itself, and will seal the exhaust quite well for some time.

  • rufusz

Posted August 21, 2008 - 12:08 AM


The whole machine-gun popping was because of too lean idle :thumbsup:
Adjusted the carb yesterday (idle-mixture), took it for a ride, and now it only popps 1-2 times on hard decel, that's normal.

  • spc mike

Posted August 21, 2008 - 09:35 AM


Yea the popping is due to beign too lean, i would just turn your fuel screw out a 1/4 of a turn.

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