It starts then stalls immediately

This is the most common explanation for 'starts then stalls.' On startup, the engine runs on the fuel already present in the carburetor's float bowl; so it starts normally for a second or two. But if the jets (particularly the idle jet) or the small passages of the idle circuit are clogged, fuel is no longer delivered continuously: as soon as that initial reserve in the bowl is used up, the engine dies from lack of fuel. The classic cause is winter storage: fuel left in the float bowl evaporates and leaves a sticky varnish that clogs the tiny calibrated passages. That's why this symptom typically shows up coming out of winter or after a long period sitting. Additional clue: if the engine keeps running as long as you hold a bit of throttle (the main circuit then takes over) but dies as soon as you return to idle, the idle circuit is clearly at fault. Cleaning it (removing the float bowl, removing and cleaning the jets with carb cleaner and compressed air, never with a wire) is intermediate-level but manageable for a careful home mechanic. Prevention is simple: drain the float bowl before long-term storage and use fresh fuel, possibly with a fuel stabilizer.

The choke (enrichener) enriches the mixture for cold starting. Mismanaging the choke frequently explains a start followed by a stall, in two opposite situations. First situation, when cold: the engine needs the choke to start and keep running for the first few seconds; if you don't use it, the engine starts on the float bowl reserve then stalls from lack of a rich enough mixture while still cold. Second situation, when warm: a choke left open makes the mixture far too rich, which can flood the engine and cause it to stall after starting, especially once it's warmed up. In both cases, checking is free and immediate: check the position and proper operation of the choke control. On a manual choke, the lever or knob should engage fully when cold and return fully when warm, with a cable that slides freely. On an automatic enrichener, check its power supply and operation. This is one of the first causes to rule out, since it costs nothing to check and often gets fixed with correct usage or a cable adjustment. If the engine runs as long as you hold a bit of throttle but stalls at idle when cold, insufficient enrichment is a strong suspect.

Many carbureted bikes have an automatic vacuum fuel tap: an internal diaphragm, subject to intake vacuum through a small hose, only opens the fuel path when the engine is running, and closes it when stopped (to keep the carburetor from flooding when the bike is off). When this diaphragm is punctured or worn out, the tap no longer opens enough: the engine starts on the fuel present in the float bowl, then stalls once it empties, since it's not being resupplied. The 'starts then stalls' symptom is therefore typical of a failed vacuum fuel tap. Clue: if holding a bit of throttle isn't enough to prevent stalling (the engine dies anyway once the bowl empties), the tap is a strong suspect. Confirmation is simple on taps that have a PRI (prime) position, which opens the fuel path by gravity regardless of vacuum: if the bike runs on PRI but stalls on ON, the diagnosis is confirmed. Testing and replacing it (a diaphragm via a kit, or the whole tap, or fitting a compatible manual tap) are intermediate-level. Also check the condition of the tap's built-in screen/filter, often clogged, and the vacuum hose, which shouldn't be punctured or disconnected.

The tank needs to let air in as fuel leaves it, to make up for the volume lost. Depending on the model, this venting happens through a small vent built into the cap or through a dedicated vent hose. If this passage gets blocked (dirt, an insect, a pinched or kinked hose, a clogged vent, a swollen cap seal), a vacuum gradually builds up in the tank and eventually stops fuel from flowing down to the carburetor or pump: the engine starts and runs for a bit on the available fuel, then falters and stalls. The symptom can show up after a few seconds or a few minutes, the time it takes for the vacuum to build. The confirmation test is immediate and free: loosen the tank cap slightly; if the engine stops stalling (and you sometimes hear a 'hiss' of vacuum releasing), the vent is indeed the cause. This is a basic cause, easy to clear (cleaning the vent or the hose). Caution: riding with a loose cap is only a diagnostic test, never a lasting fix, since a poorly closed cap lets in rain and can let fuel leak out. On newer models, a clogged evaporative emissions (EVAP) system can be involved.

On most newer bikes, an electronic immobilizer requires a transponder built into the key to be recognized by a unit linked to the ECU. If the key isn't recognized (a failed transponder, a faulty reading antenna around the ignition switch, a disrupted control unit or ECU, too-weak a battery), the system may allow a very brief start — the time it takes the ECU to verify — then cut ignition or fuel injection: the engine runs for a second then dies. The telltale sign is a key or lock warning light that comes on or blinks on the dashboard. Clue: if the engine stalls no matter what you do (holding throttle changes nothing) and the key warning light is on, the immobilizer is the prime suspect. This cause is rarer than fuel-supply problems, but real, particularly after a battery change, a damaged key, or a loss of synchronization. Diagnosis requires the manufacturer's scan tool, and getting it running again (resetting or re-learning the key) involves the security electronics: this is expert-level work, for a professional or dealership. Before blaming this, always rule out a weak battery and questionable grounds first, which can cause misleading electronic symptoms.

Repair to be left to a professional.