Hello, from USA. while I believe what you have there is a "Gasoline" model 100. The part number is not available separately here in the states. It is attached by bolts to the rear muffler as shown in the photo I posted. It is a "resonator weight" to muzzle any vibrations or noise from the exhaust system. I know because my 100 series here had the same problem, so I replace it completely with a new unit.
note" that the weight is does not come with the new muffler from what i understand
hope that helps.
option #10
part# 17403-50151
option #9
part#17405-50030
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