I received this mower from my brother in law it would not crank. I found that it needed a new starter. Replaced the starter and it starts but only now the mower is constantly backfiring. Any thoughts on what to check now. I have replaced a fuel line, cleaned the carburetor, adjusted the valves to .004" and nothing seems to help. Should I change the muffler ? I just can't make sense of this issue at this time. The mower has not ran for three to four years.