XR600R / 650L automatic decompressor...

2 replies to this topic
  • ShaiDanny

Posted April 24, 2004 - 09:13 PM


After looking into this mechanism, both in the XR600R's manual and while holding it in my hand, I've come to a conclusion:
What the heck is going on here?

Check out the description written by Honda:
First page
Second page
I've used my best brain cells, or those that are left anyway, in figuring out how this thing works and I can't understand one thing:

They say that the spring that sits inside the decompressor cam (item 2 in the lower left drawing on page 1) is responsible for opening the right exhaust valve below 800 RPM. How so? Calling this spring "weak" is an understatement... How in the world could a spring this lame open the exhaust valve with its stiff spring? :)


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  • moto9

Posted April 24, 2004 - 11:42 PM


It looks as if it's a centerfugal opperation it is engaged so that it opperates "opening the exhaust valve when the kick starter is moved through it's stroke. then once the bike fires, above 800 rpms, the centerfugal clutch kicks in to move the decompression rocker arm in to a position where it no longer depresses the valve. the reverse decompression cam is a 1 way roller bearing that freewheels when the engine is rotating in the right direction, but if it trys to run backwards the roller bearing lockes up and opens the exhaust valve so that the cylinder can't charge, this keeps the bike from running backwards. and yes it does happen,
When I worked at a bike shop I had just finished preping a new out of the crate YZ 400 or 465, anyways I fired it up and let out the clutch to test ride it around the shop, well it took off backwards and took out my roll cabinet scared the hell out of me.
The decompression gizmo looks pretty straight forward??

  • ShaiDanny

Posted April 25, 2004 - 01:22 AM


Wait, Wait, I've got it!!! :)
It seems that the little spring in question is not the part that presses against the rocker when the decompressor is engaged.
When looking closely at the bottom middle drawing in page 1, one can see that the pin (item 3) sits under a little groove that prevents the decompressor cam from disengaging. Now that's smart.

My my, those Japanese engineers... :D

Now, does anyone have any experience in modifying a camshaft to accept the decompressor assembly?
I have a Hotcams Stage 1 cam for my 650L and it doesn't use the stock decompressor assembly (no holes on the cam).
Along with the high compression piston, the electric starter has a VERY HARD time starting the engine.
Any ideas?

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