Runs rough or stalls

Misfires, hesitation, stalling at idle

This free guided diagnosis asks up to 7 questions about what the bike is actually doing and narrows it down to 8 possible faults, each with a safety call, what it costs to fix, and the guide that fixes it.

Start the guided diagnosis

What causes runs rough or stalls?

Fouled or worn spark plugs

A sooty, wet, or worn plug cannot fire reliably, so the engine misses at idle and stumbles under load. The plug is cheap to replace — but it is also a report card, so read it before you throw it away.

Ride with caution. Replace them before your next longer ride. A bike that misfires can also stall at the wrong moment in traffic. Typical cost: $10-60 DIY, $60-250 at a shop.

Dead or intermittent cylinder — plug cap, lead, or coil

A cold header means that cylinder is not burning fuel. The usual cause is ignition: a cracked plug cap, a chafed lead, or a coil that breaks down once heat opens up an internal crack. A clogged carb jet or a dead injector on that cylinder can do the same.

Ride with caution. If it is cutting out completely when hot, do not ride it in traffic until it is fixed — losing power mid-junction is a real hazard. Typical cost: $15-150 DIY, $90-400 at a shop.

Dirty or clogged air filter

A choked filter starves the engine of air, so the mixture runs rich: flat power, a lazy throttle, sooty plugs, and worse fuel economy. It rarely causes a sharp misfire on its own — but it is cheap and often overdue.

Safe to ride. Not dangerous, but replace it at your next chance — it is costing you power and fuel every mile. Typical cost: $15-50 DIY, $50-150 at a shop.

Vacuum leak — cracked intake boot or loose clamp

Air sneaking in past the carb or throttle body is not metered, so the engine runs lean. It shows most at idle, where the leak is a big share of the air the engine is taking in: a hunting idle, stalling at junctions, and hot running.

Ride with caution. Fix it soon. A lean engine runs hotter, and stalling at junctions is a hazard in its own right. Typical cost: $15-120 DIY, $90-350 at a shop.

Stale fuel or varnished carburettor jets

Petrol left in a carburettor goes stale and leaves a varnish that blocks the tiny pilot jet first. That is why the classic symptom is a bike that only runs on choke and will not idle.

Ride with caution. Sort it before you rely on the bike. An engine that stalls whenever the throttle closes will leave you stranded at a junction. Typical cost: $10-80 DIY, $150-500 at a shop.

Fuel starvation — clogged filter, weak pump, or blocked tank vent

At idle the engine needs very little fuel, so a partly blocked filter or a tired pump keeps up. Under sustained load it cannot, and the engine fades and surges as if it is running out. On carburetted bikes, a blocked tank vent or a sticking float valve does the same thing.

Ride with caution. Deal with it before a longer ride. Losing power under load can leave you stuck while overtaking or climbing. Typical cost: $10-200 DIY, $80-600 at a shop.

Carbs or throttle bodies out of balance

Each cylinder on a multi-cylinder bike needs to pull the same vacuum at idle. Out of balance, the cylinders fight each other: a lumpy idle, a snatchy throttle just off closed, and extra vibration at low revs.

Safe to ride. Safe to ride. The fix is maintenance, not an emergency — do it when you have an afternoon free. Typical cost: $30-80 DIY, $80-250 at a shop.

Failing sensor on a fuel-injected bike

The injection computer relies on sensors to decide how much fuel to add. A faulty throttle position sensor causes hesitation and a lumpy idle. A faulty coolant or air temperature sensor causes bad cold starts or a rich, rough engine when hot. Reading the stored fault code is faster and cheaper than swapping parts on a guess.

Ride with caution. Get the code read before you buy anything. If the FI light is flashing or the bike stalls in traffic, stop riding it until it is diagnosed. Typical cost: $0-150 DIY, $100-400 at a shop.

Guides that fix these faults

Other symptoms