WR/YZ 450 - Exhaust cam strange mechanism

10 replies to this topic
  • sabin

Posted November 02, 2002 - 11:47 AM


Is this the autodeco?

Posted Image

  • tk421

Posted November 02, 2002 - 12:25 PM


It sure looks like it. Reminds me of compression release on the Kohler engines. If so, that concept has been around since the late sixties or early seventies.

  • AZWR426

Posted November 02, 2002 - 03:30 PM


If not the auto-decompression release then maybe it has cam timing that varies with rpm?

  • yamaha.dude

Posted November 02, 2002 - 06:32 PM


Nice pic, If it is not the Auto Decomp, then it may be a way of varying the cam timing by single degrees, rather than a whole tooth at a time... to index it for proper performance and response to a certain power requirement... you know to get it dialled in somewhere between WR and YZ timing, one degree at a time...


  • sabin

Posted November 02, 2002 - 07:53 PM


Maybe this is where it comes the so_much_power of the 450??? I'll repost this in the YZ forum. They can look in theyr service manual and will tell us for sure...

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Posted November 03, 2002 - 06:15 AM


That is the decompression mechanism. I first saw that set up on a 96 KLX 650, but someone was claiming Kohler used it much earlier than that. Yamaha has been using it on many bikes and atvs for a while now.

It is a simple system with an excellent track record for reliability.
The exhaust cam has a shaft that runs through the middle, and rotates back and forth about 20 degrees. The shaft's movement is controlled by the weights you see on the sprocket.

At RPMs less than idle speed the weights are retracted and the position of the shaft presses a small nub up. the nub bumps the exhaust valve open every time the engine rotates. it opens the valve enough to reduce compression during part of the compression stroke. The valve is not open long enough to completely kill compression, just knock it down a little.

When the engine fires and starts to spin faster, the weights fling outward and pivot the shaft enough to retract the nub and restore full (12:1 still?) compression.

It is very simple, and there is no real added complexity when adjusting valves.

What I want to know is how they get that motor to run with those big parts of it cut away like that.

  • John_Lorenz

Posted November 03, 2002 - 07:22 AM


Its obviuos, its a defect the intake is missing one.


Just kidding :)

Looks like part of the old piston return spring See no piston in the pic

yama.dude nailed it, It sure does look more like a variable degree what-cha-ma-callit

  • yamaha.dude

Posted November 03, 2002 - 09:25 AM


Those cut-aways are a factory racing trick... to save weight... and make it easier to inspect the engine between races... :) :D

What we all want to know is how we can retrofit an 03 250 or 450 exhaust cam onto our 400 & 426 bikes...

YAMABEN, you will go down in history if you can get us the part number matches for such a swap...


  • John_Lorenz

Posted November 03, 2002 - 09:43 AM


I might be mistaken but did we not have a post of A Chad reed Pic in Washington riding with no header just a bare exhaust.
Those factory riders get all the trick stuff, I beat anything that the picture above will be on Chads bike for 03.

As far as the retro fit, Im not sure but I think that may void ..... here it comes

I would check with Yamah first


Posted November 04, 2002 - 04:16 AM


That thought did cross my mind, and I'll think I could figure it out by simply installing the 450 ex cam in my 2000 426.

The cams are different part numbers, of course, but if they have the same base circle the 450 cam might fit and run. It might just run kinda funny (maybe poorly) if it has a totally different grind on the lobe. Also too, I suspect the titanium valves have a different grind than steel valves, because titanium should be able to open and close faster due to lower reciprocating mass (not as big a problem for the 01/02 426). This fact is basically why I chickened out on putting titanium valves in my 2000.

There are companies that weld and regrind cams, and I bet a nickle that such a company could regrind the 450 cam to the specs of your old cam (including bearing journals if they are a different diameter). I believe that would work, but the length and sprocket dimensions of the cams have to be identical too.

OK, OK you talked me into it. I'll try to figure it out. My first step will be to find a company that will regrind a 450 cam to my specs (feel free to help me with this). Next step will be to order in a 450 cam and do some measuring.

Try not to get your hopes up though, It wouldn't surprise me in the least to find that the cams have some little difference that precludes installation in a 426 head.

And if EGO doesn't quit putting the word warranty in his posts, I'm going to keep all my findings a secret.

  • yamaha.dude

Posted November 04, 2002 - 09:57 AM


Good on you Ben, I figured if you have access to the parts fiche, or have them lying around... you may be able to do a comparison...

I can see it now, in two years we will still be talking about the YAMABEN mod... LOL

Don't worry, my bike is out of warranty now, anything I do to it is of my own undertaking and I will be responsible for the consequences... I would like a simple mod, either drop in or with just a little machining... otherwise it may become easier to swap the whole head over from a 450...

Looking forward to any positive results...



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