03 YZ450F exhaust valve advice needed

4 replies to this topic
  • smokingtz

Posted February 14, 2012 - 08:24 PM


I bent both exhaust valves in a freak accident with a sticky auto decompression cam. anyway, got it apart and seats and guides look good. have about 110 hours on bike and it ran fine. should i just buy a couple new valves and keep ring or do other stuff like springs, guides, etc... while it's apart? (piston was done at 100 hours). Would I lap the valves in like a conventional steel valve? Any thing else to consider? Bike sees motocross track play riding as I like to call it. Would like to keep it running safely for another 2 seasons or so. I put about 20 hours a year on.

  • grayracer513

Posted February 14, 2012 - 08:48 PM


NEVER lap titanium or hard coated steel or stainless valves.

Any time you replace a valve, the seats have to be recut/reground professionally in order for the valves to seat and to last a reasonable length of time. The fact that you bent yours raises the possibility that you damaged the guides, too, so an experienced and competent motorcycle engine machinist should evaluate them for you.

Who told you the valves being bent had anything to do with the auto decompression system?

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  • smokingtz

Posted February 15, 2012 - 09:17 AM


I told myself! I crashed the bike and my knee pushed in the valve cover gasket over the cam humps thus lodging rubber gasket material into the exhaust cam decompression eccentric thus keeping the auto decompression "on" every revolution. I kicked the bike over and it eventually fired up. Unfortunately the damage was done before i realized the gasket. I had the service manager from the local Yamaha dealer look at it. He thought I should replace the valves and guides but kind of implied that I could lap the new valves in. Didn't sound right to me cause of the titanium. I guess I'll buy some valves and guides and have a machine shop grind the seats. Thanks.

  • grayracer513

Posted February 15, 2012 - 11:12 AM


In the first place, anything dragging against the cam sprocket would cause the weight to be moved out, disengaging the auto decomp pin the same as it does when running more than 750 RPM. Secondly, the AD system is "on every revolution" when it works normally, and the pin does not lift the valve far enough to contact the piston in any event. In the third place, the AD system only bears on one of the two valves, so even if it could have bent one, it cannot have damaged the other.

The valves were bent by the timing chain letting the cam slip out of time.

  • smokingtz

Posted February 22, 2012 - 07:17 PM


although it didn't appear that the cam was out of time it probably was as both valves are bent. as for the cam sprocket. the gasket material lodged between the auto decomp weight and the sprocket and didn't allow it to move outward. i ordered valves/seats/guides/springs/keepers and will have a machine shop install them. thanks

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