Flooded motorcycle engine: how to clear it and start it

A flooded motorcycle engine has too much fuel and not enough air in the cylinder to ignite, so it cranks but will not start. The classic fix is to turn off the fuel, open the throttle fully, crank the engine for several seconds to push out the excess fuel and air it out, then try again without the choke. If that fails, pull the spark plugs, dry or replace them and crank to clear the cylinders.

This is about too much fuel, not water. If your bike has been in floodwater, read our post on motorcycle flood damage instead.

Signs of a flooded engine

Sign	What it means
Cranks normally but will not start	Too much fuel in the cylinders
Strong smell of gasoline at the exhaust or airbox	Excess unburned fuel
Wet, fuel-soaked spark plugs	Fuel has coated the electrodes
Fuel dripping from the carburetor overflow tube	Float or needle valve stuck open
Starts for a second, then dies	Mixture too rich to sustain
Black smoke or a gassy smell after it finally starts	Rich mixture clearing out
Worse after repeated starter attempts	Each crank adds more fuel

A bike that does not crank at all is a different problem. See motorcycle clicks but won't start. If you are not sure whether it is flooded, fuel, spark or compression, our broader guide on why a motorcycle won't start can help.

Why motorcycles flood

• Too much choke or enrichment. Leaving the choke on after the engine is already warm, or too much on a warm engine, pours extra fuel in.
• Excessive cranking. Repeated attempts on a cold, weak or fouled engine keep adding fuel without ignition. A weak battery makes it worse because the engine turns slowly.
• Pumping the throttle on a carbureted bike. The accelerator pump squirts fuel into the intake each time.
• Stuck float or leaking needle valve. The carburetor bowl overfills and fuel passes into the engine even when it is not running. Our guide on how to clean a motorcycle carburetor covers it.
• Fouled or weak spark. If the spark is poor, the fuel does not burn and builds up. See motorcycle spark plugs.
• Leaking injector on a fuel-injected bike, or a sensor fault that causes too much fuel.
• Restarting a hot engine too soon on some designs, where fuel boils in the bowls or lines.
• Petcock leaking into the carburetors while parked.

How to clear a flooded carbureted engine

Work outdoors or in a well-ventilated area, with the engine cool. No smoking or open flames. Check your owner's manual first, because procedures vary.

Method 1: Air it out

1. Turn the fuel petcock to off, if it has one.
2. Move the choke or enrichment lever to the off position.
3. Open the throttle fully and hold it there.
4. Crank the engine for several seconds, then stop, and repeat once or twice. Do not crank for long periods, since this strains the starter and battery.
5. Release the throttle, turn the fuel back on and try starting with a small throttle opening. Do not use the choke.
6. If it starts, let it run until it is stable. Expect some white or black smoke as the excess burns off.

Letting the bike sit for 15 to 30 minutes first can help the fuel evaporate, particularly in warm weather.

Method 2: Pull and dry the spark plugs

If Method 1 does not work:

1. Turn the ignition and fuel off. Let the engine cool.
2. Remove the spark plug caps, and use the correct plug socket to take the plugs out.
3. Check them. Wet, black or gas-smelling plugs are a flood sign. Dry them with a clean cloth, or compressed air if you have it. Replace them if they look fouled, worn or cracked.
4. With the plugs out, turn the fuel off, open the throttle fully and crank the engine for a few seconds to push fuel out of the cylinders. Fuel may spray from the plug holes. Keep your face away, cover the holes with a rag and do this away from sparks and ignition sources. Do not leave the plug caps loose where they can spark near the holes (on many bikes the starter will not turn with the kill switch off, so the caps need to be grounded or the ignition disabled instead). Some manuals say how to ground plug leads safely, so follow that rather than guessing.
5. Reinstall the plugs to the specified torque. Do not overtighten. Refit the caps.
6. Turn the fuel on and start the engine with no choke.

If the plugs are fouled repeatedly, there may be an underlying problem. Gas in the oil can also result, so check the oil next.

How to clear a flooded fuel-injected engine

On many injected bikes, the system adjusts the fuel on its own, and flooding is less common. Still, the engine can flood if you crank repeatedly without starting. A few general steps, subject to your manual:

1. Turn the ignition off and wait a few minutes.
2. Open the throttle fully, and crank the engine for several seconds. Many fuel-injected bikes cut fuel delivery when the throttle is wide open during cranking, which is the "clear flood" mode, but not every model does this. Check your manual.
3. Release the throttle and try starting normally. Do not pump the throttle.
4. If it still will not start, pull and dry the plugs as described above.
5. A bike that floods repeatedly may have a leaking injector, a failing sensor or a spark problem and needs diagnosis. See our check engine light guide.

Check the oil afterward

Raw fuel can wash past the rings into the crankcase and thin the oil. After a flood event, pull the dipstick or check the sight glass. If the oil level has risen, it smells strongly of gasoline, or it looks thin and watery, change it before running the engine hard. Our guides on how to change motorcycle oil and what oil your motorcycle needs will help. Do not keep cranking a bike if you suspect liquid in the cylinder. A cylinder full of liquid does not compress, and forcing it can bend parts. If the engine suddenly stops turning or feels locked, stop.

How to avoid flooding

• Use the choke only as long as the engine needs it, and follow the manual's cold-start procedure.
• Do not pump the throttle on bikes where it is not called for.
• Limit starter runs to a few seconds, and rest between tries.
• Keep the battery charged. See how to charge a motorcycle battery.
• Replace worn spark plugs on schedule. A tune-up helps, see motorcycle tune-up cost.
• Turn the petcock off for storage, and use a fuel stabilizer. See how to winterize a motorcycle.
• Keep the carburetors clean. Old fuel varnishes the float needle.
• Check for fuel leaks. See motorcycle leaking gas, which is also a fire hazard.

Typical repair costs

Costs vary by bike, region and shop. Clearing a flood yourself costs nothing, while an underlying fault does not.

Repair	Typical range
Spark plugs	About $10 to $50 each, plus labor
Oil change after fuel dilution	About $60 to $200
Carburetor clean or rebuild	About $100 to $500
Float needle or float replacement	Often part of a carb service
Injector cleaning or replacement	About $100 to $600 or more
Diagnosis	Billed hourly, see our diagnostic cost guide

When to see a mechanic

Call a shop if:

• It still will not start after clearing it and drying or replacing the plugs.
• It floods again each time you try to start it.
• Fuel is leaking from the carburetor, a hose or the petcock.
• You smell gasoline in the oil, or the oil level is rising.
• The starter strains, the engine feels locked or you hear a knock.
• You are not comfortable removing spark plugs or working near fuel.

A shop can check the float and needle, injectors, spark and compression. Look at diagnostic shops or motorcycle maintenance shops, or browse motorcycle repair shops in Minnesota and a city such as Minneapolis.