There could be several reasons why a motorcycle won't start after sitting for a while with fuel in the tank. Here are some common issues to consider:
Stale Fuel: If the motorcycle has been sitting for an extended period, the fuel in the tank may have deteriorated or become stale. Fuel can degrade over time, leading to the formation of varnish and deposits that can clog the fuel system. This can prevent the engine from starting or running properly. In such cases, draining the old fuel and refilling with fresh fuel might solve the problem.
Fuel System Issues: The fuel system components, such as the fuel pump, fuel filter, or carburetor (or fuel injectors in modern motorcycles), can become clogged or blocked if the motorcycle sits unused for a long time. Contaminants or residue in the fuel can accumulate in these components, affecting fuel flow and preventing the engine from starting. Cleaning or servicing the fuel system may be necessary.
Battery Issues: If the motorcycle's battery is weak or discharged, it may not have enough power to start the engine. When a motorcycle sits idle for an extended period, the battery can lose its charge. Try jump-starting the motorcycle or charging the battery to see if that resolves the issue. If the battery is old or damaged, it might need to be replaced.
Ignition System Problems: Issues with the ignition system, such as faulty spark plugs, ignition coils, or spark plug wires, can prevent the engine from starting. Over time, spark plugs can wear out or become fouled, affecting the ignition process. Checking and replacing any faulty ignition components might be necessary.
Mechanical Problems: There could be mechanical issues that are preventing the engine from starting, such as a seized engine, faulty starter motor, or problems with the valves or piston rings. If you suspect a mechanical issue, it is best to have a professional mechanic inspect and diagnose the problem.
It's worth noting that these are general possibilities, and the specific cause may vary depending on the motorcycle's make, model, and condition. If you're unsure about diagnosing or fixing the problem yourself, it's recommended to consult a qualified motorcycle mechanic for assistance.