Lets call the knob with the spring under it the mixture screw because...well... thats what it is.
Believe I asked you to richen it (counter clockwise) 1/4 - 1/2 turn. Lets do this again and start the engine.
This should tell us that the carb is probably dirty but again, one step at a time.
Now, since your carb is open at the front I want you to do something for me at the point the engine wants to slow and stall.
Take your first and second fingers and turn them around so the backs of them are facing the carb opening and just lightly block the opening momentarily and see if the engine pics back up. Practice it before you start the engine. You will feel a definite "pull" on the backs of your fingers but don't worry.....it won't swallow your fingers!