The end of the stock Yamaha molded plastic throttle tube is "capped" for the lack of a better word.
You must, at the very least, cut that molded cap off, or drill a large enough hole in it to accept the handguard mounting hardware.
Not sure why yamaha used such a crappy throttle tube.
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2008 WR250X
Gearing: 13t - 48t
Power Commander 5 / PC-V
Airbox Door Removed - Flapper glued - AIS removed
FmF Q4
Bridgestone Battlax BT-003rs