Beware of Auto decomp damage



108 replies to this topic
  • pablo83

Posted April 18, 2009 - 06:26 PM


From the WR FAQ:

Q: How many link pins are between the cam marks for YZ timing?
A: If you have a 400 or 426, you will need to rotate the exhaust cam sprocket 1 tooth clockwise at TDC. This will give you 12 pins between the marks, stock is 13.

Here's my cams with stock timing. On mine there are 14 pins between cams. What's going on?

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  • grayracer513

Posted April 20, 2009 - 10:37 AM


From the WR FAQ:

Q: How many link pins are between the cam marks for YZ timing?
A: If you have a 400 or 426, you will need to rotate the exhaust cam sprocket 1 tooth clockwise at TDC. This will give you 12 pins between the marks, stock is 13.

Here's my cams with stock timing. On mine there are 14 pins between cams. What's going on?

Posted Image

What's going on is that the info you quoted has nothing to do with your bike. That's for a 400/426, like it said. Those bikes did not have auto decompression cams, so you could move the cam between YZ/WR specs without affecting anything else. All that changed in '03, since when you change the timing of the exhaust cam, you also change the timing of the auto decompression events.

Where the "12:00" marks are on the cams has no real meaning, except where one is fudging something, as when using am OEM auto decomp cam in an older model, etc. It can be useful as an assembly aid, or as a cross check when the real timing marks don't seem to line up right, but otherwise, they don't matter. The important thing is that the cams are each correctly timed to the crank, not to each other.

  • pablo83

Posted April 20, 2009 - 02:21 PM


I guess I should have stated that this is an '01 WR426 with a decomp cam installed. The PO says it's an '03 WR cam, but he knows very little about what he had done to the bike. Is there some way I can verify what year cam this is?

  • grayracer513

Posted April 20, 2009 - 04:19 PM


I guess I should have stated that this is an '01 WR426 with a decomp cam installed. The PO says it's an '03 WR cam, but he knows very little about what he had done to the bike. Is there some way I can verify what year cam this is?

Yes, that does make a difference, alright. In that case, IF the cam was originally made to be used in a WR or YZ 450, the only correct way to time it is to set the timing on the intake cam as per the book, then space the exhaust cam top mark 14 pins from the top mark on the intake.

If the cam was an aftermarket unit made for earlier engines, it's a different story. In that case, you revert to using the regular marks, and what I posted above applies.

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  • pablo83

Posted April 20, 2009 - 07:03 PM


Thanks. It starts and runs good that way. I advanced it one pin clockwise for "YZ timing" because I thought the same timing strategy would work on the decomp cam. The decomp part worked fine, but the engine would not start.

  • grayracer513

Posted April 20, 2009 - 08:43 PM


Thanks. It starts and runs good that way. I advanced it one pin clockwise for "YZ timing" because I thought the same timing strategy would work on the decomp cam. The decomp part worked fine, but the engine would not start.

One tooth clockwise retards the cam 22 crankshaft degrees. It also delays the reseating of the exhaust valve by the decomp pin by the same 22 degrees, leaving the engine with too little compression to run.

  • pablo83

Posted January 25, 2010 - 01:11 PM


Just a followup on this thread... When I looked at my auto decomp bucket last year it was all beat up like the others in this thread, so I installed a new one. I checked the valve clearances recently so I removed the auto decomp bucket to inspect it. It looks perfect. The bike has about 2000 miles on it since the bucket was replaced.

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  • Southwick

Posted March 28, 2014 - 02:05 PM


Just found this old thread, and since I'm rebuilding the engine of a friends WR450, I've come across the same damage as described here. For those interested I thought I'd add the info that the pin in question can easily be removed from the  exhaust camshaft by simply unthreading the bolt which secures the counterweight to the gear. This way, with the pin removed, any polishing/rounding of the tip of the pin will be a breeze, and damage to the cam lobe avoided.

 

/Lasse



  • Road Rage

Posted March 30, 2014 - 11:12 AM


My Auto decomp  mechanism in the cam  gummed up and would stick in the retracted position, caused excessive load on my starter and eventual starter fail, perhaps your auto decomp mechanism  is gummed up and the pin won't retract when it should??


Edited by Road Rage, March 30, 2014 - 09:38 PM.





 
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