WR426 Exhaust cam mod

9 replies to this topic
  • SPDFRK450

Posted October 26, 2004 - 07:58 AM


Can anybody gimme a quick rundown on the exhaust cam mod for the 426's. I was looking on the thumperfaq website, but I am not sure if all that info applies to the 426 as well. I know 99% of that site does, but not sure about this. Also is it just an adjustment, or is the exhaust cam mod the complete changeout of a WR cam for a YZ cam. And for my final question does anybody know where I can see online service manuals. I know the yamaha site wont let you see the service manuals online. Thanks guys.

  • jchantzWR400F

Posted October 26, 2004 - 09:08 AM


Well, to answer your first question, yes the cam mod applies to the 400's and 426's. It is a cam swap with the 03 cams with auto-decompression. If you want YZ timing, get the 03 YZ cam. If you want WR timing, get the 03 WR cam. Now, if you are talking about the advancement of timing using your stock exhaust cam, then it's a matter of adjustment, not replacement. Many have done both adjustment and replacement with success.

  • SPDFRK450

Posted October 26, 2004 - 11:44 AM


I hope I am not sounding ignorant, but I was thinking that since I have an 02 426, then I would get the 02 YZ exhaust cam if I needed to replace it. You are suggesting the 03 so I will then have automatic decompression? My understanding from the thumperfaq website is that since I dont have automatic decompression on mine that I can just rotate my exhaust cam one tooth clockwise to achieve the YZ exhaust timing.

  • BrandonW

Posted October 26, 2004 - 01:26 PM


You are correct. If you just want the YZ timing, you can rotate your current exhast cam to get that performance.

If you want the decompression (HIGHLY recommended) then you need to decide which timing you like, because you cannot switch the decompressioned cam back and forth between WR and YZ.

Hope that helps you out.


  • jchantzWR400F

Posted October 26, 2004 - 01:30 PM


Like Brandon said, if you simply want YZ timing, advance your stock exhaust cam by 1 tooth in the timing chain.

If you want auto decompression and YZ timing, you can purchase the 03 YZ exhaust cam for around $100. Either way, you'll be happy with the modification.

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  • SPDFRK450

Posted October 26, 2004 - 08:45 PM


Thanks for the insight fellas. I think I will just rotate the current cam I have in there 1 tooth. Other than that is there anything I need to watchout for, or tips you fellas have otherwise?

  • jchantzWR400F

Posted October 27, 2004 - 06:35 AM


When taking the timing chain off of the cam, be sure and keep it tight (upwards) in order to keep it on the crank. If it slips on the crank, you will have to retime the intake cam, before you can set your exhaust cam. I let this happen to me when I went through the process. Other than that, just be careful with the cam cover gasket, it should be reused. Also, make absoulutely sure that the engine is at TDC. It helps if you put the bike into 5th gear when doing this. Otherwise, it doesn't want to stay at TDC. Just some tips... :cry:

  • SPDFRK450

Posted October 27, 2004 - 11:17 PM


But how about readjusting the valves on the exhaust cam. I am new to this kind of stuff so If the question I just asked was stupid just let me know. I figure that if I lift the actual cam off the valve itself that I will have to readjust the exhaust cam or at least check it.

  • jchantzWR400F

Posted October 28, 2004 - 06:22 AM


OK, not exactly sure what you're asking. If you're asking if you need to readjust your valves after doing this mod, you don't. You are only trying to advance your exhaust cam timing 1 tooth. This does not affect valve clearance. Now, it is a good idea to check clearance since you're in there already. If your clearances are out of spec, simply find a place to get shims from and adjust to within manual spec. If your bike is currently starting and running fine, chances are, your valve clearances are in spec. These Yamaha's seem to be very easy on the valves.

  • SPDFRK450

Posted October 28, 2004 - 07:14 AM


Sounds good. Today is the Exhaust valve project day. Lets hope my next post is a good one huh.


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