Mountfield Omega 42 mixture too rich?

My 10-year old Mountfield Omega 42 lawnmower's fuel mixture seems too rich: engine smoking, spark plug carbonised, smelss of unburnt fuel, engine floods easily under load. Engine starting and idling fine, air filter clean. The only time it runs well is the 20 seconds before it runs out of fuel ... Any ideas how to adjust it? Note push-prime, no choke, automatic throttle (via airflow from flywheel) and doesn't seem to have adjustment screws on carburetor.