How to fix: Leak in the fuel system: how do you go about it?

Safety first: a fuel leak is a major fire risk. Don't ride or start the engine until it's fixed, keep any flame or spark source well away, and work in a properly ventilated space. Diagnosis confirmed: fuel is leaking from the fuel system, causing the level to drop without riding, giving off a fuel smell, and leaving marks on the ground. Common causes: a cracked or poorly clamped fuel line, a hardened carburetor float bowl gasket, a fuel tap that's weeping, a float needle valve that no longer seals (the bowl overflows through the vents), or a loose fitting. The fix: pinpoint exactly where the leak is coming from, with the fuel tap closed, then replace the faulty part or gasket and tighten or replace the clamps. This is an intermediate-level job, manageable for a careful, cautious home mechanic. Risk level: moderate mechanically, but high on the safety side (flammability) -- hence the emphasis on precautions. Cost: a fuel line and clamps cost a few euros; a float bowl gasket / needle valve kit runs roughly 15 to 40 euros; a complete fuel tap around 20 to 70 euros (check pricing). Never ignore a leak, even a small one: a fuel line weeping today can fail tomorrow on a hot engine. If you're unsure of the source, or if the leak is coming from the tank itself, hand the job to a professional. This tool helps you understand what's going on -- it doesn't replace a certified mechanic.

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Smell of petrol / fuel

You smell a strong fuel odor while riding, at a stop, or when entering the garage. Warning — deal with this as a priority: gasoline and especially its vapors are highly flammable, and a leak near a hot engine, a spark, or a flame is a real fire risk. Never ignore a persistent fuel smell. The main causes: a leak in the fuel system (a cracked hose, a loose fitting, a leaking fuel-tap or carburetor float-bowl gasket — you'll see damp marks), a blocked tank vent (the tank can no longer breathe, pressure fluctuates, fuel off-gasses, and you hear a 'hiss' when opening the cap), a carburetor flooding (a float/needle valve that no longer closes, fuel overflowing the bowl — especially on 2-strokes), or, on a fuel-injected bike, a leaking injector (fuel dripping into the combustion chamber). You can tell them apart by inspection (is there a visible damp mark?), the fuel-cap test (a pressure 'hiss'?), and timing (smell at rest in the garage vs. while riding/starting). Urgency threshold: high from a fire-safety standpoint. If you see an active fuel leak (dripping), don't ride and don't start the bike until it's fixed, work in a ventilated space away from any flame, and store fuel properly. A fuel smell is never 'normal': find the source and fix it quickly. Three simple checks steer the diagnosis: a visual inspection (do you see a damp mark?), the fuel-cap test (a pressure 'hiss'?), and when the smell is strongest (at rest in the garage, or while riding).

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Possible causes: smell of petrol / fuel

These are the most common causes, from most to least likely. The guided diagnosis above helps you find yours, test by test.

Leak in the fuel system
Intermediate
This is the most common and most serious (fire risk) cause of a fuel smell. The fuel system (tank, tap, hoses, carburetor or injector rail) can leak at several points: a cracked or loosely clamped fuel hose, a loose fitting, a weeping fuel-tap seal, a hardened carburetor float-bowl gasket, or a worn needle valve that lets the bowl overflow. The result is a damp fuel mark, sometimes a drip, and a strong smell, especially at rest. This is a hazard: fuel dripping near a hot engine or a spark source can ignite. The tell: a visible damp mark on the hoses, carburetor, or fuel tap, and a noticeable fuel smell at rest. The repair principle: locate the source (fuel tap off), replace the faulty hose/gasket/needle valve, and retighten the clamps. This is intermediate-level work, to be done in a ventilated space, away from any flame. This cause overlaps with the fuel leak behind the higher-fuel-consumption symptom. Hard rule: don't ride and don't start the bike until the leak is fixed, and never ignore a weeping hose (it can fail outright on a hot engine). Even a minor fuel leak should be dealt with immediately.
Blocked tank vent
Basic
The tank needs to be able to breathe: a vent (in the cap, or a dedicated hose) lets air in as fuel goes out, and equalizes internal pressure as temperature changes. If this vent gets blocked (dirt, a pinched hose, a clogged vent, a fouled EVAP emissions system on newer models), pressure inside the tank is no longer regulated: in hot weather, fuel expands and off-gasses, building up pressure that pushes vapors out (smell in the garage), and you hear a distinct 'hiss' when opening the cap. Conversely, as fuel is used up, a vacuum can form and starve the fuel supply (the engine stalls — see the 'starts then stalls' symptom). The tell: a fuel smell mainly at rest/in the garage, and a clear 'hiss' every time you open the cap. The repair principle: clear the vent (cleaning the cap's vent or the vent hose). This is basic work, free or nearly so. This cause overlaps with the tank-vent cause behind the 'starts then stalls' symptom. It's distinguished from a leak (no damp mark here, but a vapor smell and pressure buildup). A blocked vent is common and easy to fix.
Carburetor flooding (float or needle valve)
Intermediate
Inside the carburetor's float bowl, fuel level is kept constant by a float and a needle valve (like a toilet cistern): as the level rises, the float rises and the needle valve shuts off the fuel supply. If the needle valve is worn/fouled and no longer seals, or if the float is punctured (waterlogged with fuel) or misadjusted, the level rises too high and fuel overflows — through the passages, the overflow, or weeping from the carburetor: 'the carburetor floods.' The result is a strong fuel smell at rest, hard starting (a flooded engine), black smoke on startup, and sometimes fuel dripping out. This is common on carbureted 2-strokes and 4-strokes, especially after winter storage (the needle valve gummed up by varnished fuel). The tell: a strong fuel smell at rest with a damp mark at the carburetor, and an engine that floods. The repair principle: remove the bowl, check/clean the needle valve and its seat, check/adjust the float (replace if punctured), and clean the bowl. This is intermediate-level work. This cause overlaps with rich carburetion and fuel-system leaks. Fire hazard: an overflowing bowl lets fuel run out — shut off the fuel tap, work calmly in a ventilated space, and don't start the engine while it's flooding.
Learn about the part: Carburettor →
Leaking injector
Expert
On a fuel-injected bike, injectors spray fuel under pressure and must seal tightly between injections. An injector that no longer closes (wear, fouling, a fault) lets fuel drip/flow into the intake or combustion chamber: the result is a fuel smell (especially at startup and while riding), a risk of the engine flooding, hard starting, and — the danger — fuel that can pool in the cylinder or dilute the engine oil (fuel in the sump). The tell: a fuel smell mostly while riding/starting on a fuel-injected bike, with erratic starting, and no obvious external damp mark (the leak is internal, toward the engine). Diagnosis (reading live data with a scan tool, testing the injectors, checking fuel pressure) and replacing the faulty injector should be left to a properly equipped professional: this cause is rated expert-only, since it involves the injection system and fuel inside the engine is a serious risk. This cause overlaps with the check-engine-light symptom (injector). It's distinguished from an external leak (no mark on the ground) and from a tank vent issue (no 'hiss'). A leaking injector shouldn't be taken lightly, both for fire risk and oil dilution.
Repair to be left to a professional.

Leak in the fuel system

Blocked tank vent

Carburetor flooding (float or needle valve)

Leaking injector