Rough idle / the bike stalls at a stop

At idle, the revs won't hold steady: they rise and fall (the engine 'hunts'), stumble, or the bike stalls outright at lights and at a stop. Idle is a tricky operating regime, because the engine is running slowly on very little air and fuel: the slightest imbalance in the mixture or the ignition shows up immediately, whereas it would go unnoticed at higher revs. Most often, the cause lies in the air and fuel supply: a mis-set idle speed, a partially blocked idle/pilot jet (the small calibrated passage that feeds the engine at low throttle openings), a false air leak at the intake that leans out the mixture, or a fouled spark plug causing misfires. On a four-stroke engine, valve clearance out of spec can also upset the idle. It's worth distinguishing: idle instability mainly when cold that improves as it warms up points to an adjustment or light fouling; idle instability all the time points to a false air leak or a spark plug; an idle that stays abnormally high and won't come down suggests a false air leak or a throttle cable that doesn't return; an idle that sags and stalls suggests a blocked idle circuit. Context matters: a problem that appeared after winter storage almost always points to a jet fouled by stale fuel. Urgency threshold: an unstable idle isn't dangerous in itself, but stalling at an intersection or during a maneuver can be — better to diagnose it early, especially since the most common causes are free or cheap to fix.