I found you question as my neice has just bought a SRX 50 with the same problem. These guy's supply parts that should sort it out.
http://www.scooter-assassins.com/HondaShadow50Page.html
I'm thinking of trying the CDi or heavier rollers, but I don't know which will work yet. If you've any tips I'll check back!
Pete
August 2007