Powervalve on top is broken. yamaha 250yz cross.

A have a yamaha dtr 125 it runs but its got no power and the valve moves but isn't kicking what could it be thanks

rebuilding or replacing engine

Service Manual

Shut the fuel off, pull the line from the carb, then just turn the fuel onto the reserve setting.

Sounds like it is time to take the carb apart and clean or replace what needs replacing/cleaning.

i think you will find that the cables you are referring to as powervalve cables (if you are talking about the xv1900 in the picture)are not powervalve cales you will find that they are for the auto decompression valve in the cylinder head,if you follow them back you will find they terminate on a lever with a tensioner you can back off the tensioner to remove them but shouldnt have to just to remove the header,hope this helps

yes, nicasil actually

The cyl will have a letter stamped on it to indicate the piston size.