Hi stuart, as steve says-it's extra air, i can't remember the name of the valves either, they're spring loaded and are supposed to open on the overrun i think? anyway, the springs go weak and allow valves to open-you can either fit new butterflies with no valves (hard to get now?) or as i used to do, snap the valve, remove spring assy and solder the valve into the butterfly. cheers, ian(n) ps i was only fourteen when i used to fix these!