To me a misfire would be the absence of a fire, as in nothing happens when it should. A back fire is fire when there shouldn’t be as in out of time. So, when you have a backfire through the carburetor for example the ignition timing is such that the plug fires out of time while the intake valve is open causing the explosion to escape (back) through the intake and carburetor. You can also have what we call a backfire, generally through the exhaust system. This is caused by an out of tune carburetor or ignition that allows excess unburned fuel to enter and build up in the exhaust system only to be ignited by hot exhaust gases causing the customary pop or boom.
