Probably just fouled plugs from spoiled gas, might be from not being ridden and just idling, although on a 4 stroke it shouldnt matter. Might I suggest putting a teaspoon of engine oil in each cylinder before you park it for the winter. It prevents the cylinder from drying out and you wouldnt need to start it every few weeks. I usually drain my tank and leave a teaspoon of oil in the cylinder of my dirtbike. It smokes a bit when you first fire it up but runs great after a couple mins.
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