What causes a backfire
Gases in the engine exploding at the wrong time, Ignition timing set wrong, misfire putting un-burnt fuel into the exhaust system, then ignited by hot gases. Most common fault, is an ignition cross fire, where a spark plug is fired at the wrong time, happened mostly with high tension plug leads breaking down in damp conditions and shorting across each other, causing the plug to ignite gases in the cylinder at the wrong time when a engine valve is open.
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