Sitting too long is a posibility.
Years ago (2003) I purchased a small portable (Generac 1000) A really nice little traditional enclosed genny.
Out of the box the waveform was rather nasty. in fact an APC UPS objected to it.
After a tank of gas (Powering nothing more than a light) the APC was happy and a scope showed a nice clean waveform (Many portables are very nasty on the scope). sadly the genny grew a pair of legs and walked off.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times