I have non working model 420 E . On opening it up Ifound the remains of a hair thick coiled spring that goes to the governor arm from the walbro carburator.Could the broken spring be resposible for the intemitant running characteristics of the engine ? It starts then dies out after ofew seconds.

Hi,
I assume(perhaps incorrectly) that you have an MTD with an automatic choke. Yes, if you mess with it it no good and if it does not work it will affect starting.
Peace,
Clarke