Backfiring / popping from the exhaust

This is the most common cause of popping on deceleration. When the mixture is too lean (too much air for the available fuel), combustion is incomplete and slow: on deceleration, with the throttle closed, unburned fuel and poorly combusted mixture reach the still-hot exhaust and ignite all at once, producing the characteristic 'pops.' The sources of leanness: partially clogged jets on a carburetor (often the idle circuit, fouled after winter storage), or a false air leak at the intake (a cracked intake boot, a loose clamp, a tired gasket letting in uncounted air). This cause often comes with other signs of leanness: a tendency to run hot, pinging under load, sometimes a lack of pep. Diagnosis is intermediate-level: you look for a false air leak with the engine idling by spraying a volatile product around the intake boots (revs change where air is getting in), and/or you clean the carburetor's jets with cleaner and compressed air. Fixing the leanness gets rid of the popping. Important: beyond the noise, a lean mixture makes the engine run hot and can damage the valves; it needs addressing. Always work back from the manufacturer's stock specs, and never deliberately lean out the mixture.

A leak in the exhaust line, most often at the joint between the manifold and the head or at a loose clamp, can by itself cause popping on deceleration. The mechanism: on deceleration, with the throttle closed, pressure pulses appear in the exhaust; through the leak, fresh air (rich in oxygen) gets drawn into the line and lets unburned gases finish combusting in the hot pipe, hence the 'pops.' This is, in effect, a localized leaning-out right at the exhaust itself. You'll often spot a black soot mark around the leaking joint, and can confirm it by feeling a puff of gas by hand (carefully, engine idling) near the manifold/head joint. This cause overlaps with the 'exhaust noise' symptom: the same leak causes both noise and popping. The fix is intermediate-level and accessible: retorquing the fasteners and replacing the exhaust gasket if it's crushed, after cleaning the mating surfaces. Beyond the popping, a manifold leak throws off the mixture (and the lambda sensor on fuel-injected bikes) and can expose a valve to overheating — worth fixing without delay. This cause is easy to confirm (black soot at the joint, a perceptible puff of gas near the joint), making it a quick, cheap check before considering heavier causes.

A poorly functioning ignition system causes misfires: at times, the mixture in the cylinder isn't ignited (or ignited poorly), and these unburned gases then explode in the hot exhaust, producing pops — which tend to show up under acceleration or load, when the ignition system is working hardest. Causes: a failing spark plug (worn, fouled, wrong gap) producing a weak or intermittent spark, a faulty spark plug cap or ignition coil, or ignition timing that's out of spec. The spark plug is by far the simplest thing to check first: remove it, read its condition (light tan electrode = healthy) and check the gap, then clean it or replace it with the correct part (check the gap spec in the manual). This is intermediate-level and accessible work. If the plug is healthy and the misfires persist, look toward the ignition system (coil, cap, timing), which may require a professional. Popping caused by ignition issues often comes with power loss and hesitation — this is what sets it apart from simple lean-mixture popping on deceleration. Worth fixing, since repeated misfires can eventually overheat the catalytic converter on a bike equipped with one.

On a four-stroke engine, this is the most serious cause of popping. The exhaust valve must seal perfectly against its seat to keep the chamber airtight and to cool itself through that contact. If it no longer closes (valve clearance left too tight and unadjusted, a worn seat, a deposit), it stays exposed to hot combustion gases, overheats, and eventually burns: part of its sealing surface disappears. The seal is then broken, and on every cycle, still-burning or unburned gases pass through to the exhaust, producing pops, especially under acceleration and load, along with a clear power loss (that cylinder's compression has collapsed). It's the combination of 'popping plus significant power loss' that points to this cause, as opposed to simple leanness (where it pops but still pulls). Confirmation comes from a compression and leak-down test, and the repair requires removing the cylinder head, replacing the valve(s), checking/lapping the seats and guides, and reassembling to torque spec: this is an engine repair rated expert-only, left to a professional. Prevention: keeping up with valve clearance checks avoids the most common cause (clearance left too tight and never corrected).

Repair to be left to a professional.