I am the original owner of an ´85 650SC Nighthawk. Rode it today in fact. Your fuel consumption is way too high. My bike gets 62-65 MPG. It also will accelerate to 65 MPH in seconds and would run much faster than that if I turned it on. Don´t know what´s ailing yours. Can you check the compression? Is there much variation between the cylinders? Could one or more of the carbs need rebuilding?