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A pound to a penny it is stale petrol. Drain the tank completely and rinse with a little fresh petrol. Remove float chamber to both access the main jet for removal and cleaning, and also for thorough cleaning of the float chamber. If feeling really thorough you could actually remove the whole carb for cleaning….. The gummy residues that this so-called ‘petrol’ leave behind can be a swine to remove from the smaller primary airways in a carburettor.
Clean spark plugs and check the gap. Also check that both are sparking well. Put some new fresh Super Unleaded in the tank, and try to start the engine. If you still get no noise let us know.
Modern petrol, with ethanol and all sorts of other chemicals in it, has an absolutely maximum life of about six weeks in the tank before going “off”, and causes corrosion of nearly everything in the fuel system of a vintage bike or car. You can buy various additives to keep it under control. Also if the bike is standing unused for even a short period, stick a piece of pvc tape over the vent hole in the filler cap, to stop the more volatile petrol ingredients from escaping, but don’t forget to remove it before starting the bike!
P.S. In the sort of hot weather that we are currently having, the petrol in your tank will go “off” even more quickly, as the ‘aromatics’ do their best to evaporate, and it is those very same low-boiling fractions of the petrol that give easy starting….