Early this spring I picked up a clean 05 yz, the guy I bought it from had recepts from his local yam dealer showing the top end was rebuilt just a few weeks before. The bike had good compression and ran strong. I had been riding an xr 400 so the yz felt like a beast. I rode it through the summer.
Raced in 5 local woods races. loved the bike but it always seemed kinda soft on the bottom. t weeks ago i got a chance to ride a buddys yz125 and it had more grunt and low end than my 250. hmmm. I figured my rings were shot as I guessed I had about 80 hours on the top end.
Pulled it apart and the piston and rings were nearly in spec, bore looked good. Then I looked at the POWER VALVES, the guy at the dealer had installed the 2 valves for the side booster ports BACKWARDS with the slot facing inside. This held them in the open position and bound them up in the cams that operate them effectively locking them in the full open position. Put the valves in the correct way and got them to operate correctly, installed the fresh Wiseco. After a proper break-in, WOW its like a whole different bike.
Check a used bike out real close.
