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So I have a 06 Yz450f. And I was out riding the other day when the bike suddenly died I rolled to a stop thinking I was out of gas. Upon inspection found there was plenty of fuel I went to start it again I kicked it over slowly trying to find the compression stroke and nothin low compression but still some compression I popped it in gear and rolled it backwards thinking maybe the auto decompression deal in the exhaust cam got stuck for some reason and still nothin... if I twist the throttle while kicking there is absolutely no compression I can push the kicker with my hand. The bike was running great before all of this and it kinda came out of nowhere. I know valve shims are notorious on these things but I never heard of them just going all at once. 

Edited by Brigham Richardson

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I certainly wouldn't rule out valve shims, but that usually doesn't happen instantly.

Timing chain may have jumped.

Pull valve cover and crank timing plug, and check timing marks.

Edited by WALKINGWOUNDED
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Ok I checked the timing everything looks good. I guess I gotta go get a new feeler gage and check valve clearance. Why does it completely loose compression when I open the throttle?

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With the throttle open, you are letting more air in the problem area or letting more air escape the problem area.

If the intake valves are being held open, or even just tight, with the throttle closed, you have a closed area that can let the piston pull against the throttle slide (vacuum), and push against the throttle slide (compression), so you can feel it in the kick starter.  With throttle open, the piston will pull and push the air past the throttle slide and you'll have less or no compression.  I'm running that through my mind in video form for whatever that's worth.  That's all I got.

As you mentioned, check valve clearance next.

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so I finally got back to working on the bike today and after pulling the cams I found a broken intake valve. Huge bummer but I pulled the head and nothing is damaged the piston got a little scuff on it but I would call it fine considering it's less than a year old. My question is do I just buy the one valve and get it repaired? What are the odds of the other valves doing the same thing?

Edited by Brigham Richardson

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