How can stop my mower petrol running rich?

My Flymo Quicksilver 46SD petrol lawnmower is running rich - leaving soot on the mower body and gobbling up petrol. It runs in a series of bursts, revving up and accellerating, and then slowing down again, so the governor is working, even when it is not under load.
the mower doesn't appear to have a conventional carburettor, and I can't see any jet to adjust.