07-28-2015, 11:33 PM
Last time we were up north riding, the YZ250 spit out it's piston. Of course in typical 2 stroke fashion, the pieces then took out the cylinder and crank. Once we had it all apart, I noticed that a small piece had also broken off of the end of the power valve. Unfortunately, the valve itself is only available as a complete assembly for $180, so I decided to repair it myself. The part that failed was a 4.5mm diameter pin machined on the end of the aluminum valve, and the reason it failed was that it was machined with a sharp inside corner that fractured from the vibration of the motor. A surprisingly poor design for Yamaha. The fix was pretty easy. I machined off the end of the valve, drilled an reamed a hole in it and pressed in a steel piece that I made to replace the broken section, then cross drilled it for a dowel pin and machined the offset pin on the end with a boring head in the mill. The hardened steel wear piece pressed over the pin just like factory. The difference is that the pin is now steel with a proper radius in the corner for durability, so it should last the life of the bike.
Tom
Tom