I know the engine you are speaking about and if you have to put a screwdriver in the carb then you have the auto choke system on you carb........
2 things can be wrong..... 1st your spring came loose are is broken or just plain gone.....
2nd your bi metal alloy by the muffler is shot... That rarely happens though.... Check the spring and the plastic vane to make sure it's free and can move.......
Let us know Mon Ami ~!~!