Why is my motorcycle sputtering when I accelerate?

When you open the throttle the engine needs more fuel and a stronger spark. A clogged jet or filter, a weak fuel pump, an air leak, or worn plugs and coils often show up as a stumble or sputter under acceleration.

Can bad gas make a motorcycle sputter?

Yes. Stale fuel, water in the tank and fuel with more ethanol than your bike is designed for can all cause sputtering. Draining old fuel and refilling with fresh gas of the right type is a cheap first step.

Is it safe to ride a motorcycle that sputters?

Mild sputtering at idle is usually not urgent, but sputtering that leads to loss of power or stalling at speed is a safety risk. Avoid traffic and highways until it's fixed.

Does a dirty air filter cause sputtering?

It can. A badly clogged filter restricts air and makes the mixture too rich, which can cause sputtering, sooty plugs and poor performance.

Motorcycle sputtering: causes, checks and how to fix it

A motorcycle sputtering is an engine that isn't burning fuel evenly: some combustion events are weak or missing. The cause is almost always a fuel supply problem (dirty carburetor, clogged filter, bad gas, weak pump), an air problem (leak or clogged filter), or a weak spark from worn plugs, caps or coils. When and how it sputters points you toward the right one.

Match the sputter to the cause

When it sputters	Most likely causes
At idle, smooths out with throttle	Clogged idle jet, air leak, low idle speed, fouled plug
When you open the throttle	Lean mixture, clogged main jet or filter, weak pump, weak spark
At steady highway speed	Fuel starvation (filter, pump, tank vent, petcock), ignition breaking down under load
Only when cold	Choke or cold-start enrichment problem
Only when hot	Heat-sensitive coil or sensor, fuel vapor
After sitting or refueling	Stale fuel, water in fuel, wrong fuel
In the rain or after washing	Water in plug caps, wiring or connectors
Sputters and backfires	Lean mixture, exhaust leak, timing

Fuel problems

Bad or stale gas

Fuel that sits for months in a tank or carburetor goes stale and leaves varnish. Water can get in through condensation or a bad cap. Both cause sputtering. Start with fresh fuel.

Fuel type matters, too. E15, a 15% ethanol blend sold at some U.S. pumps, isn't approved for motorcycles: the EPA lists on-highway and off-road motorcycles among the vehicles that can't use it. Fuel with more ethanol than your manual allows can also affect warranty coverage. Use the fuel grade and ethanol limit in your owner's manual.

Dirty carburetor

On carbureted bikes, partially blocked jets are the classic cause of sputtering. A clogged pilot jet causes sputtering at idle and low throttle. A clogged main jet or needle jet causes it at higher throttle. Our guide on how to clean a motorcycle carburetor covers the job. Multi-carb bikes may also need a sync afterward.

Fuel injection problems

On injected bikes, look at:

Dirty or clogged injectors that spray unevenly. See motorcycle fuel injector problems.
A weak fuel pump that can't keep pressure up under load.
Sensors such as the throttle position, temperature or oxygen sensor giving bad readings.

Restricted fuel flow

Clogged fuel filter or tank screen
Kinked or collapsed fuel line
Faulty vacuum petcock or leaking vacuum hose
Blocked tank vent: sputtering that gets worse the longer you ride, then improves when you open the gas cap

Air problems

Intake leaks. Cracked intake boots or loose clamps let in air the fuel system doesn't account for, causing a lean sputter, often at idle.
Clogged air filter. Restricts air and makes the mixture rich. See how to clean a motorcycle air filter.
Exhaust leaks. On injected bikes, an exhaust leak ahead of the oxygen sensor can cause wrong fueling. On any bike, exhaust leaks can cause popping.
Aftermarket parts like a new exhaust or intake without re-tuning can leave the engine lean.

Ignition problems

A weak spark causes misfires, which you feel as sputtering. Common causes:

Worn spark plugs. As the electrode edges erode, the spark gets weaker and less reliable, and the engine loses power. Plugs past their service interval are a common cause.
Fouled plugs. Carbon or oil deposits on the plug tip can cause a loss of spark energy and misfiring. Oily plugs point to oil getting into the combustion chamber.
Cracked plug caps or wires, especially when wet
Failing ignition coils, which may break down when hot or under load
Loose or corroded connectors

Plug condition tells you a lot. A light brown tip generally means normal operation. Black and sooty means rich. Oily means oil is getting in. See motorcycle spark plugs for how to read them.

Electrical and charging

A failing stator or regulator can drop system voltage. As voltage falls, the ignition and fuel pump weaken and the engine sputters, often with dimming lights. Test charging voltage at the battery with the engine running.

How to fix a sputtering motorcycle

Fresh fuel. Drain old gas and refill with the correct grade.
Air filter. Inspect, clean or replace.
Spark plugs. Pull and inspect. Replace if worn or fouled.
Plug caps and wires. Check for cracks, corrosion and secure fit.
Fuel flow. Inspect the filter, lines, petcock and vacuum hoses. Try riding with the gas cap loose briefly in a safe area to test the vent.
Intake boots. Look for cracks and loose clamps.
Charging voltage. Check at idle and higher rpm.
Carburetor or injector service if the basics don't solve it.
Fault codes on fuel-injected bikes.

Is it safe to ride?

Light sputtering at idle that clears up with throttle is mostly an annoyance. Sputtering that causes power loss or hesitation at speed, or that leads to stalling, is a safety risk because the bike may not respond when you need it to. Stop riding and get it checked if it sputters and dies, backfires heavily, or you smell fuel.

Typical repair costs

Costs vary by bike, region and shop.

Repair	Rough range
Spark plugs (installed)	About $40 to $250, more on bikes with hard-to-reach plugs
Air filter	About $20 to $100 plus labor
Fuel filter	About $20 to $150 plus labor
Carburetor cleaning (single)	About $100 to $250
Multi-carb clean and sync	About $250 to $600 or more
Injector cleaning or replacement	About $100 to $500 or more
Ignition coil	About $75 to $300
Fuel pump	About $200 to $600 or more

Common mistakes when chasing a sputter

Adding fuel additives instead of finding the cause. A cleaner can help mild varnish, but it won't fix a cracked boot, a worn plug or a weak pump.
Changing several things at once. If the sputter goes away, you won't know which change fixed it. Work one item at a time.
Turning the idle screw up to cover an idle sputter. That masks a lean idle circuit or air leak.
Ignoring the gas cap. A blocked vent is a cheap fix that's easy to miss.
Re-jetting before checking basics. Jetting changes rarely fix a stock bike that used to run well. Something changed: look for it first.
Mixing up plug heat ranges. Use the plug the manual specifies. A plug that runs too cold can foul and misfire, and one that runs too hot can overheat.

Also keep a short log: when it sputters, the fuel level, the weather and what you changed. That makes the shop's job faster and cheaper, and it helps you see patterns you would otherwise miss, such as a sputter that only shows up after a fill-up at one station.

If you recently fitted an aftermarket exhaust or air filter, consider that the bike may need a fuel map or jetting change to match. A dyno shop can measure the mixture across the rev range and confirm.

When to see a mechanic

Get professional help if:

The sputter causes stalling or loss of power at speed
You've done fuel, filter and plugs and it persists
A warning light or fault code is present
You suspect an injector, fuel pump, coil or sensor
The bike backfires heavily or smells strongly of fuel

A shop that does motorcycle diagnostics can test fuel pressure and spark quickly. You can find one in the motorcycle repair directory.