It uses too much fuel

You're filling up much more often than before, your range has shrunk, or you're calculating a fuel consumption per 100 km well above the model's normal figure. Excessive fuel consumption almost always comes from a mixture that's too rich: the engine is getting too much fuel relative to air, and the excess fuel is wasted. Two mechanisms cause this richness: either there's not enough air (a clogged air filter choking the intake), or there's too much fuel (carburetion or fuel injection out of adjustment, jets too large, float level too high, a choke stuck on, a failed O2 sensor on fuel injection). A separate cause, not to be overlooked: a fuel leak, which lowers the level even without riding, with a characteristic smell and marks on the ground — needs urgent attention since it's a fire risk. Signs that go along with excess richness: black smoke from the exhaust, a strong fuel smell, a black and dry spark plug, fouling, sometimes a power loss. The approach is to rule out causes from simplest to most complex: start with the air filter (the most common and easiest) and the choke, before carburetion, and save electronic diagnosis (O2 sensor) for last. Urgency: excess consumption alone isn't dangerous, but it's costly and signals an engine that isn't running right (premature wear, fouling, a risk to the catalytic converter); and a fuel leak, on the other hand, calls for stopping immediately. Better to diagnose it early: the most common cause is free to fix.