Like many others, I don't run my generator a lot but am religious about excercising it at least once per month. This spring, I couldn't get it to start even though I had run it the month before and it turned over fine when I hit start switch. Decided to check usual suspects - gas and spark. Was getting gas to the genny, had almost full fuel tank so decided to check spark. When I reached under to pull plug wire, the wire came out in my hands - approx. 6" of plug wire with the connector. Looking it over, it became quite obvious that an animal (squirrel or mouse) had chewed thru the plug wire and further investigation revealed a huge nest on top of the genny. $650 later I had a new plug wire installed along with other wiring that was messed up - an expensive event. The good news - Insurance covered it !!
Kadus to GMAC for including rodent damage under comprehensive. Moral of story - if all else fails, check for animal damage. I now keep small tray of mothballs in my generator compartment and have seen no further instances of nest building.