Treacle, Engines Need to Be Adjusted When The Seasons Change Or When You Change Fule Brands.First Try Resetting The High Speed Needle First.A worn Prop Can Be A Problem.Just Hitting The Ground Ounce Can Cause Problems.
Avistar 2 Also Made A Good Sugestion.When In Dought Change The Plug.Metal From Brake In Can Fowl The Plug But Still Start The Engine.
A Small Air Leak In The Fuel Line (Bubles In The Line To the Carb) Will Stall the Engine But Should Be Checked.
Good Luck And Start Simple Then Try The Harder Things To Fix The Engine