my money says it's leaning out because your carb is clogged... its because of the crap ethanol.
I let one of my bikes sit for about a month... I failed to get all the fuel out of the carb before I let it sit. it did exactly what you're talking about for two days until it cleaned itself out.
I always turn my petcock off about a block away from my house so that it dies from fuel starvation. i never have any problems...