Backfiring / popping from the exhaust

Sharp 'popping' sounds come from the exhaust (most often on deceleration, when you let off the throttle), or, more rarely and more violently, a backfire occurs on the intake side (a 'pop' in the carburetor or airbox). Either way, it's a case of poorly controlled combustion: mixture burning in the wrong place or at the wrong time. On deceleration, unburned fuel builds up and finishes burning in the hot exhaust, hence the popping. By far the most common cause is a mixture that's too lean (clogged jets, a false air leak at the intake, a cracked intake boot), sometimes made worse by an exhaust leak that draws in air there too. More rarely, a mistimed ignition or a failing spark plug causes misfires whose unburned gases explode in the pipe, and — the most serious case on a 4-stroke — a burnt exhaust valve lets ignited gases through, along with a clear loss of power. Telling them apart by timing is valuable: popping on deceleration points to a lean mixture or an exhaust leak; explosions under acceleration/load point to ignition or a burnt valve. Power level also helps: it pops but still pulls fine = mixture/exhaust; it pops AND clearly lacks power = valve or ignition. Urgency threshold: occasional popping on deceleration is rarely serious, but a backfire on the intake side can ignite the air filter (a hazard), and a burnt valve requires an engine repair. Worth diagnosing to prevent it from getting worse.