not sure i have issue with something or not so here's the set up now

vnl1 cases ground out and matched to a pinasco magny cours cylinder
squish is on the large side 2.37mm 1.5mm cylinder base packer (i tried a .5 packer to bring the squish down but the compression went over 150 psi)
compression 130psi
60 mm crank pinasco
22 mm dellorto si vortex
jetting ac 160
mixer be-3
idle jet 55/160
main jet 122
exhaust sip road 2 twin tail pipe
it goes like the clappers but has bad vibration at high revs and it feels like it will explode
so what is happening when riding when i shut the throtle there is no usuall distinctive vespa noise the engine sound mute like its not running is this normal
any pointers much apprieciated