B12 is pretty strong stuff, but simply adding a can without running thru the whole tank is not a cure.
Running completely thru the tank is a better temporary fix. No chemical on the market will disolve minute rust particulates tho, and if this is what is blocking the circuit, it's pretty much a pull the rack and re-do thing when the bike sits.
As for running the bowls dry, that really doesn't work either, it just allows "lowering" the fuel level in the bowls to a point no more fuel is picked up, and there will still be lots of gas there, with the sad understanding that it will evaporate sooner, leaving the particulate matter there in the end, only to be carried into the circuit on the first usage again, in even higher concentrations...blockage is emminent.
Start by completely removing the fuel from the tank, and sucking all the fuel from the bottom corners using a turke baster with a hose attached to it...repeat flushing a couple times till you are sure there is no more rust. you will still need to clean the circuits tho, there is little you can do to prevent clogging when the bike sits so long. Sorry
if storing, always make sure the tank is completely filled with fresh good gas, to prevent condensation. Do not open the tank unless you are filling it, this just allows more moisture in there if it's a partially filled tank.
In lieu of adding B12, start adding a marine stabilizer if it's gonna sit, use the Marine grade Stabil product, it's designed to work better with ethanol laced fuels.
oh, and no matter what you add to the gas, do not add multiple "combinations", this just exacerbates problems, and causes bad results as each additive needs to be run completly out, before adding a different one....don't play chemist with the fuel, the outcome is never good.