2003 660 Raptor: has spark, fresh fuel, introduced starting fluid in carb, even removed spark plug and put fuel in the cylinder and replaced plug and it wont fire. Any ideas out there what to try next?

 

If you got good spark and fuel in cylinder then you need compression to make an boom... IMO I would check compression... Did this atv just quit? are you an new owner of this atv? Has atv been having problems starting for anwhile? May have to adjust values first then we go from there,if compression is low. an good condition engine compression on an 660 should be at least 100 pounds BUT the ideal would be 150-180 pounds....