WR/YZ 450 - Exhaust cam strange mechanism
Posted November 02, 2002 - 12:25 PM
Posted November 02, 2002 - 03:30 PM
Posted November 02, 2002 - 06:32 PM
Posted November 03, 2002 - 06:15 AM
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.
Posted November 03, 2002 - 07:22 AM
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
Posted November 03, 2002 - 09:25 AM
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...
Posted November 03, 2002 - 09:43 AM
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
" WARRANTY "
I would check with Yamah first
Posted November 04, 2002 - 04:16 AM
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.
Posted November 04, 2002 - 09:57 AM
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...