Chainsaw Repair

the tank is only one step. if it had crap gas in it, the carb needs to be properly cleaned and rebuilt as well.
if the fuel went bad, or had time to harden into varnish, the fuel line and filter need to be replaced. not just clamped off and flushed.
to me, using smaller nuts/washers is a better idea. they fit into the smaller areas and will break up any hardened varnish better. these tanks are flimsy on the best of days, best not to have 2 'heavy' nuts banging around in there. especially if the gas has gone that bad. it has a way of affecting the plastic and the seam is the first place for problems to start.
CHANGE the fuel line. you WON'T get it clean.
properly clean the carb. don't use high pressure from an air compressor on it. low psi only. I only use plastic safe (sensor safe) carb cleaner or NON-chlorinated brake cleaner. if it doesn't say 'NON- chlorinated' clearly where you can see it on the label. it isn't safe for the carb.
you are also going to need a new carb mounting gasket.
since the saw has sat for quite a while, how's the intake boot and impulse grommet? this is a very known issue area for these saws. I won't ask about the carb mounting block threads......

IIRC it is 3x5 mm fuel line. The HVA part number is Husqvarna 544324902 (replaces 501768301)