Wr450 Cam In Yz

12 replies to this topic

Posted January 11, 2006 - 01:17 PM


I Keep Hearing About This Auto Decompress Cam From Wr450 (i Think)
Can Anyone Explain How This Actually Works In Technical Terms Please?
Why Dont You Save A Bunch Of Cash And Be A Man And Learn How To Kick The Yz Properly!
No Offence!

  • Indy_WR450

Posted January 11, 2006 - 01:27 PM


The YZ 450 also has the auto decomp cam. Both the WR and YZ 450s have had them since 2003! :applause:

  • grayracer513

Posted January 11, 2006 - 01:44 PM


The WR has milder cam timing than the YZ, which is something to bear in mind if you want to calm the bike down quite a bit. In choosing this cam for an earlier bike, decide which cam timing you want up front, because it's impractical to re-time an Auto Decomp cam as you can with the old ones.

The way that the Auto Decompression system works is that there is, in the hollow shaft of the exhaust cam, a shaft that engages a pin in a cross drillway. As this sub shaft rotates, the pin is moved in or outward in its bore. The outside end of the sub shaft connects to a spring loaded flyweight mounted on the exhaust cam sprocket. At speeds below idle, the spring overcomes the flyweight and pulls it inward. That rotates the sub shaft, which in turn extends the decompression pin. As the cam rotates and the engine approaches BDC at the beginning of the compression stroke, the extended pin comes into contact with the left exhaust valve lifter and raises the valve off its seat slightly, preventing the engine from building any compression. As the engine gets to about 60 degrees or so before TDC, the decompression pin is rotated out of contact with the exhaust cam, and the valve reseats, allowing compression pressure to build for the remainder of the stroke. In this way, what would be around 200 pounds of cranking compression is reduced to something more like 120, enough to start the engine, but little enough that a small starter motor, or your leg, can crank it through without help.

Once the engine starts, the flyweight overcomes the return spring and rotates the sub shaft to retract the pin, and the valve train operates in a normal, ordinary manner.

  • BergArabia

Posted January 12, 2006 - 12:03 AM


Why Dont You Save A Bunch Of Cash And Be A Man And Learn How To Kick The Yz Properly!
No Offence!

I have found that kicking the YZ for me is kind of like a good golfing stroke. For me at least, when I relax and just follow through it works best, with no sweat and no problems. Actually I am really happy about it now coz in the last few weeks for the first time I can start the bike from a sitting position smooth and easy as butter. BTW I am 5' 5" 70kg and 49 years old. If I can do it anyone can do it...

  • Fizz

Posted January 12, 2006 - 06:01 AM


what is most likely to break down first, the manual decomp. or the auto decomp.?

i can buy a used (not much though) -04 450 cam for a fairly small amount of money, but reliability is number one for me, and i dont mind using the decomp. handle, i dont race, and theres newer a hurry to get the bike started again ...

  • ncmountainman

Posted January 12, 2006 - 06:36 AM


supposedly the yfz cam is timed somewhere between the wr and yz. is that an auto decomp also? i would assume that it fits,anyone ever try it?

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  • Matty05

Posted January 12, 2006 - 07:22 AM


YFZ autodecomp too!

My neighbour runs a YZ450 ex cam in his quad, maybe I should try his out.........

  • grayracer513

Posted January 12, 2006 - 07:40 AM


Why Dont You Save A Bunch Of Cash And Be A Man And Learn How To Kick The Yz Properly!

Because, following a crash, I can kick my 450 4 times in the time it takes you to set and kick a manual engine once. Besides, it came that way.

And it's offense, old chap.

  • Birdie426

Posted January 12, 2006 - 09:55 PM


It's not just about the kicking...the autodecomp cam mod also allows you to bump start and eliminates the stall when you stuff in hard into a corner about three gears to high and don't get on the cluth quick enough. Oh yea...and "compression braking"...what's that? It makes a great bike all the more enjoyable to ride.

  • grayracer513

Posted January 12, 2006 - 10:02 PM


Oh yea...and "compression braking"...what's that?

It's the resistance of a four cycle engine to "dead spinning" that results from the pumping ation that takes place during coasting in gear. My factory auto decompression 450 has a ton of it, and I like it like that. Auto decompression does not function on coast, or at any speed as high as a slow idle. Sorry.

  • Birdie426

Posted January 13, 2006 - 09:53 AM


Gartracer513 -
I noticed a significant reduction in compression braking when I put the hotcams autodecompressor cam in my 426. It still has some, but it's alot more like a pinger now.

  • grayracer513

Posted January 13, 2006 - 01:20 PM


Maybe, there ae a number of things that will affect the amount of engine braking. But it has absolutely nothing whatever to do with the auto decomp feature. It would be mechanically destructive to have it engage while running.


Posted January 13, 2006 - 03:39 PM


Thanks for the response just what i was looking for Grayracer!
We need more guys that understand 4 strokes properly.
Ill stick with my YZ426 cam although I would be interested in any advice on the heavier flywheels. I have a magneto on mine to power road light etc. does this in itself make the flywheel heavier ( I assume it does, so how then do you go about fitting a heavier flywheel, or is it a waste of time).
Many thanks

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