Loosing Gas

[RossWal@domain.elided]

My 2 bbl 345 has come up with a new problem. If it sits for more than a week 
or two (I haven't narrowed it further, 1 week is OK -  2.5  is not) it looses 
all the gas in the bowl. A slosh from a can down the throat & it starts & 
runs great, but without that, you can crank till the battery runs down. I 
have not yet tried to diagnose this, but though someone might have seen the 
problem before. 

Thanks for any thoughts! 
Ross Walsmith 
73 Scout II