Mike
Firstly is the cold start lever stuck in the on position (ie pushed over to the left towards the driver)? If not then if it were me I would whip the decks off (if not already) remove the carb, and give it a general poke/ clean to see what that does. In my experience once you have accepted the need to remove the decks it is not so big a job, and makes everything hugely easier....
Look forward to hearing progress.
Tim