Could be petrol after just a month. If there wasn't a lot in the tank to start with it would have lots of air to help it go stale. I've got a Montessa trials bike that's carbed and it doesn't like starting on petrol that age. It's a pretty crude old nail with magneto ignition so it's a marginal starter in cold weather anyway (just like the SS's according to Keefyboy) so it might not take much to be the straw that broke the camels back.
Jools
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