I was told several years ago when I wanted to buy a atv to stay away from the 660 engine because of heating problems. I bought a Grizzly 550 at that time. I am looking for a used Yamaha atv for my wife now and see several used with the 660 engine. Is there some years to stay away from because of the heating problem ? I would hope that Yamaha has fixed the problem on the later models but still wonder why they seem to ask less for ones with that engine. Thanks .