Well you sucked me in, I had never heard of an Over Erving mower. Apart from that, can you get to the throttle and check that it is closing at low setting?
If it is a full crank engine a dodgy decompressor can cause over revving as well as a crank case air leak or blocked jet. If it's a power torque it could be the top starter oring which is common does the engine respond to throttle at all ? Or does it just scream the same all the time