I have an older Montgomery ward tractor mower that won't fire. I got the thing for free due to the fact they didn't want to fix it. I replaced the ignition switch, spark plug, oil, fuel lines and filter and put a new battery in it. When I turn the key it chugs as if it were going to start but never does. I'm am out of ideas so please help. It has a briggs and stratton 12 hp motor