bogging acting like choke is on

[dwint51]

my scooter has been running great then I think I might of got some premium gas with ethonal.this is whats happening.its sputtering like the choke is on or a bad plug till I hit a down hill and once it gets to 8000 rpms shes fine and will do over 50.i have a bbk,pipe and jetted to a 80.my pipe always wakes up at 8000 rpms but it cant get there unless I get a down hill and once it hits that mark it will scream until I let it go below the 8000 rpms.it starts right up also but sputtering till those rpms.changed plug also