Pinking / knocking noise under acceleration

Octane rating measures fuel's resistance to self-igniting under pressure. Every engine is designed for a minimum octane rating, based on its compression ratio and ignition advance. If you use fuel with too low an octane rating for your engine (for instance, 91 octane on a bike that needs 93/98, or poor-quality fuel), it ignites spontaneously before the spark under demanding conditions (high load, low revs, heat): that's knocking. This is by far the most common cause and, fortunately, the simplest and cheapest to fix. The telltale clue: knocking that shows up right after a fill-up at an unfamiliar, little-used gas station, or with questionable fuel, points directly to this. It's generally enough to refill with the octane rating recommended by the manufacturer (check your manual), possibly diluting a bad tank with higher-octane fuel. Fuel stored too long in a can also loses octane and can cause this. This is the first thing to try since it's free, risk-free, and often enough on its own. If the knocking disappears with the next fill-up, the diagnosis is confirmed.

A mixture that's too lean (too much air relative to fuel) burns hotter than a correct mixture: this rise in combustion temperature favors pre-ignition and therefore knocking, especially under load. Sources of leanness: partially clogged jets on a carburetor (not enough fuel getting through), a parasitic intake air leak (a cracked boot, a loose clamp, a tired seal letting in uncounted air), or, on fuel injection, a faulty sensor. This cause is trickier than octane, since a simple fill-up won't fix it: the fuel system or intake needs to be diagnosed and repaired. A lean mixture often comes with other signs: backfiring on deceleration, a tendency to run hot, sometimes a lack of pep. Finding an air leak is done with the engine idling while spraying aerosol carb cleaner around the intake boots (the idle speed changes wherever air is getting in); cleaning the jets restores proper fuel delivery. This is an intermediate-level job. Important: a prolonged lean mixture isn't just a noise issue — it overheats the engine and can damage valves and the piston — it needs fixing, especially since it compounds the effects with the knocking.

Carbon buildup is the collection of sooty deposits that accumulate in the combustion chamber and on the piston crown over the miles, especially on an engine often run at low revs, fed poor-quality fuel, or with a bit of oil making its way into the chamber. These deposits have two effects that favor knocking: they reduce the chamber's volume, effectively raising the real compression ratio, and they form glowing hot spots that can ignite the mixture before the spark (pre-ignition and pre-heating). The resulting knocking is chronic and independent of the last fill-up, which sets it apart from a simple octane problem. Treatment ranges from accessible (a decarbonizing fuel additive, and an 'Italian tune-up' — regularly running the engine up through the rev range safely to burn off some of the deposits) to more technical (professional top-end cleaning, or even mechanical decarbonizing during a cylinder head removal, reserved for a pro). This is an intermediate-level job. Prevention matters: quality fuel, avoiding prolonged very-low-rev use, and keeping up with oil changes (oil that seeps in feeds carbon buildup). Worth fixing, since hot spots sustain damaging knocking.

Ignition advance is the moment, slightly before top dead center, when the spark fires, so combustion pressure builds at the optimal time. It's managed by the ignition system (CDI unit or ECU) based on revs and load, using a crankshaft position sensor. If the advance is too great — a maladjusted ignition on older mechanically-timed systems, a faulty position sensor, a failed ignition module, or a wrongly modified map (reflash, aftermarket ignition box) — combustion starts too early and its pressure fights the rising piston: the result is chronic knocking, present as soon as the engine is under load, regardless of fuel. This is a less common but serious cause, since permanent ignition-related knocking damages the engine (overheating, piston damage). Diagnosis involves the high-tension ignition system and electronics: it requires a timing light (on adjustable systems) or a scan tool, checking the sensor and the ignition module, and manufacturer data. That's why this cause is flagged expert-only, for a professional. Before considering this, always rule out the simple causes of knocking first: too-low octane fuel and a too-lean mixture, which are far more common and cheaper to fix.

Repair to be left to a professional.