London to a brick it is the float needle, the aftermarket plastic ones DO NOT work, complete waste of time. First thing I replace before trying to start any of these mowers is put in a viton tipped needle, try that and depending on what it does maybe then replace the primer cap. One important thing is when fitting a new needle is to check that the float can close the needle without hitting the tower the needle is located in. A few times I have had to grind a bit off the bottom end of the needle so the needle can drop down enough to allow fuel to flow before the float hits the bottom of the carby casing. The reason this varies slightly is how deep the seat has been set in the primer cap.
