You could also take the plate, held on by two bolts, off the side of the injection pump. Be sure to keep it extra clean as you do it.
With a screwdriver, make sure the slide (rack) moves freely against a spring backwards and forwards. It can stick and give you these problems sometimes. Use lots of WD40 to make sure it is free.
Also check that the small button in the pivot of the stop control is out. Grip it with a pair of pliers and pull. Push it in and out again with lots of WD40.
You should set the throttle half way, the button should stay in then fly out when the engine starts or the stop is pulled.
As said in other posts, make sure all your filters are clean, there is one in the fuel tap that can catch you out.