one of ours did that and it turned out that the airbleed screw was not adjusted correctly allowing a lean burn detonation at idle. Does yours shut off with the kill switch when it does this or do you have to kill it with the clutch in 1st gear?
make sure yu have gas in it.
when mine runs out of gas it starts revving realllllly high.
it pisses me off
when you run it out of gas it leans out the motor causing it to rev high.