It uses too much fuel

This is the first thing to check, the most common cause, and the easiest to fix. The air filter traps dust before the intake; as it clogs up, it lets less and less air through. But the engine runs on a precise air-to-fuel balance (roughly 14.7 parts air to 1 part fuel by mass on a 4-stroke gas engine). If air decreases while fuel stays the same — the case on a carburetor, which doesn't self-compensate — the mixture becomes too rich: the excess fuel burns poorly and is wasted, causing excess consumption, often along with a power loss, black smoke, and a fouled spark plug. On modern fuel injection, the O2 sensor partially corrects for this, but a badly clogged filter still eventually hurts both consumption and performance. Checking it is purely visual (open the housing and look), and fixing it (cleaning a foam or oiled-cotton filter, replacing a paper filter) is a basic, cheap, quick job. The maintenance interval depends heavily on the environment (dust, rain, off-road use clog it fast): check the interval in your manual. A clean filter often restores normal consumption immediately; it's one of the most cost-effective maintenance jobs on a bike.

A mixture too rich from excess fuel is a direct cause of excess consumption. On a carburetor, several settings can enrich the mixture: jets too large (often installed by mistake after an exhaust or aftermarket filter swap without rejetting), a float level set too high (a worn float needle that no longer seals, a misadjusted or leaking float, flooding the bowl), a needle set too high, or a mis-set idle mixture screw. On a fuel-injected bike, an unsuitable fuel map, a faulty sensor (temperature, air pressure), or a dirty throttle body can enrich the mixture. Accompanying signs: black smoke from the exhaust, a strong fuel smell, a black and dry spark plug, an unstable idle, sometimes a lack of pep. This is an intermediate-level cause to diagnose and fix. On a carburetor, cleaning, checking the float level, and adjusting the mixture are doable for a methodical home mechanic who always works back from the manufacturer's stock settings. On fuel injection, precise diagnosis is better left to a shop with a scan tool. Golden rule: never modify a jet or a setting at random; any abnormal richness should first have the simple causes checked (air filter, choke) before touching the carburetion itself.

Warning: a fuel leak is first and foremost a fire risk, to be dealt with as an absolute priority: don't ride until it's fixed. This is a misleading cause of 'excess consumption': the fuel isn't disappearing by being burned, but by leaking out of the system, which lowers the tank level even without riding. Common sources: a cracked or poorly tightened fuel line, a hardened carburetor float bowl gasket, a fuel tap that weeps, a float needle that no longer seals (the bowl overflows through the vents), or a loose fitting. The signs are distinctive and easy to spot: a persistent fuel smell, especially with the bike parked, and marks or drips on the ground under the bike. Unlike a rich mixture, there's not necessarily any black smoke. Diagnosis (locating the source, with the tap closed) and the fix (replacing the line, gasket, float needle, or tap, tightening clamps) are intermediate-level and manageable for a careful home mechanic, provided you work in a ventilated space, away from any flame. Never ignore a leak, even a small one: a line that weeps today can fail tomorrow on a hot engine. If you're unsure of the source, or if the leak is from the tank itself, have a professional handle it.

The choke (enrichment device) deliberately enriches the mixture for cold starting. If it stays engaged once the engine is warm — a manual choke left pulled out, a stuck cable that doesn't return, or an automatic choke stuck open — the engine keeps getting a much-too-rich mixture all the time, which can nearly double fuel consumption. This is a simple, common, and free cause to check, worth ruling out early in the diagnosis. Signs: marked excess consumption, black smoke, a high idle once warm, a spark plug that fouls quickly (black, dry electrode), sometimes misfiring and poor throttle response. On a manual choke, just check that it's pushed fully back to the riding position once the engine is warm, and that the cable returns freely (lubricate, adjust if needed). On an automatic choke, check its power supply and operation: an element that no longer closes enriches continuously and needs replacing. This is a basic check. The reflex: if excess consumption comes with an abnormally high idle when warm and black smoke that rarely eases off, think of the choke before suspecting the carburetion. While you're at it, clean the spark plug fouled by the excess richness.

On a fuel-injected bike, the O2 sensor (oxygen sensor) placed in the exhaust measures the oxygen content of burnt gases and tells the ECU the actual mixture richness. The ECU uses this to fine-tune fuel injection in closed-loop mode, aiming for the ideal ratio and thus controlled consumption. As the sensor ages, fouls, or fails, it no longer reports accurately: the ECU often switches to a safer, richer default mode, driving up consumption, sometimes with black smoke and a check-engine light (MIL) with a stored fault code. This is a cause specific to fuel-injected bikes, and expert-level: diagnosis requires a scan tool (reading live data and the fault), and other causes that mimic a failed sensor need to be ruled out first — a clogged air filter or an exhaust leak upstream of the sensor can throw off its reading. Replacement (accessing the sensor in the exhaust, sometimes seized in place, and working with the injection electronics) should go to a properly equipped professional: this cause is flagged expert-only. Never replace an O2 sensor at random without confirming the fault and ruling out the simple causes, or you risk swapping an expensive part for nothing.

Repair to be left to a professional.