06' YZ450FSPV Strange Problem Update

1 reply to this topic
  • superxking22

Posted July 05, 2008 - 07:06 PM


I tore the top end down, and checked the camshaft sprockets. They looked to be fine with just regular wear on them, no grooves or messed up teeth. I checked the cam chain to see if it had stretched any by extending the tensioner all the way out and seeing if it would bottom out agains the head. I had about an inch to work with so I took and retimed it and put it back together to perform a leak down test. I still couldn't get the bike to start so I could not warm the engine for the test.

Went ahead and tested to see what readings I got and with Piston at TDC I had a MAJOR! air leak coming through the exhaust. You could actually feel air blowing out it. It also sounded as if you could hear it through the air box. Especially if going from an open to closed throttle.

My question is does this mean I have a bent valve? Or could they just need to be shimmed? Or is it something completely different from either of those?
Any Help is greatly Appreciated.

Thanks, Cole

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  • grayracer513

Posted July 05, 2008 - 08:15 PM


If you have a leaking exhaust valve, there are only 3 possible reasons for it:

> It has a portion of the sealing face burned off or broken away. Valve clearance might be normal. This just about never happens with titanium valves.

> The valve is bent, or being held off the seat by carbon or a rusty seat. Valve clearance will be excessive.

> The valve has worn through the hard facing, and has less than zero clearance when cold. The head expands more than the valves with heat, and the valve can wear to zero in operation. When the engine then cools, the cam will be holding the valve off the seat, and you will have no compression. Valve clearance will be zero.

So, I guess your problem is all in your head.

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