[theMAG]

2 possible reasons.:

Your engine seems to be running rich. Get the carb cleaned and the float checked.

Also in carb engines there's a tiny valvepin thru which petrol is passed thru on the way to the carb. If this nozzle is clogged even with a tiny dust particle the petrol flow is obstructed and the engine starves - which if corrected by throttling will keep the engine running but flood the carb making it rich. Get this nozzle cleaned.