How can I tell if there's gas in my motorcycle oil?

The oil smells like gasoline, looks thinner than normal, and the oil level may rise between changes. A lab oil analysis can measure fuel dilution if you're not sure.

Can I ride with gas in the oil?

Not with a lot of it. A heavily diluted oil can't protect bearings properly, and a crankcase full of fuel is a fire and hydrolock risk. Change the oil and fix the cause first.

Will a little fuel in the oil burn off?

Small amounts of fuel from cold starts usually evaporate once the oil gets fully hot on a longer ride. Frequent short trips don't give it the chance, so fuel can build up.

Why does my carbureted motorcycle get gas in the oil?

The usual cause is a float needle that doesn't seal, letting fuel overflow from the float bowl into the intake and down into the crankcase, especially with a gravity-fed petcock left on.

Gas in motorcycle oil: causes, risks and how to fix it

Gas in motorcycle oil usually comes from one of two sources: a carburetor that overflows fuel into the engine (most often a stuck or worn float needle), or fuel slipping past the rings because the engine runs rich, misfires or never gets fully warm. Either way, fuel thins the oil so it can't protect the engine, so change the oil and find the cause before riding much further.

Signs of gas in motorcycle oil

The oil smells like gasoline on the dipstick or when you open the filler cap
The oil level is higher than when you last filled it
The oil looks thinner and runs off the dipstick more easily
The bike was hard to start, flooded, or smelled of fuel after sitting
Fuel drips from the carburetor overflow tube or the air box
The engine runs rich, with black smoke, fouled plugs or poor fuel economy

A slight fuel smell in the oil of an older carbureted bike after a few cold starts is common. A rising oil level is not.

Why fuel in the oil is a problem

Engine oil depends on viscosity to keep a film between moving parts. Fuel thins oil, so the oil loses viscosity and lubricates less effectively. In practical terms:

Thinner oil forms a weaker film on bearings, cams and the transmission, raising wear
Fuel can wash oil off the cylinder walls, increasing ring, piston and cylinder wear
The oil gets more volatile, so it can be consumed faster
On most bikes the transmission and wet clutch share engine oil, so shifting and clutch behavior can suffer
A crankcase with a lot of fuel in it is a fire hazard, and fuel that fills a cylinder can cause hydrolock when you try to start

Common causes

Cause	Bike type	Clue
Stuck or worn carburetor float needle	Carbureted	Fuel from overflow, flooding after sitting, rising oil level
Petcock left on or leaking (gravity feed)	Carbureted	Problem after storage, fuel in the oil overnight
Failed vacuum petcock diaphragm	Carbureted	Fuel flows with engine off
Rich mixture or stuck choke	Both	Black smoke, sooty plugs
Short trips and long idling	Both	Oil never fully warms up
Misfiring cylinder	Both	Rough running, wet plug
Leaking fuel injector	Fuel-injected	Hard hot starts, fuel smell, wet plug
Worn piston rings	Both	Blow-by, oil consumption, low compression

Carburetor float problems (the classic cause)

A carburetor float bowl has a float and a needle valve that shut off fuel when the bowl is full. If the needle sticks open because of dirt or varnish, or its rubber tip is worn, fuel keeps flowing. It overflows through the carburetor, into the intake, past an open intake valve and down past the rings into the crankcase. On bikes with a gravity-fed petcock (no vacuum shut-off) or a petcock left "on" or in "prime," the whole tank can drain into the engine overnight.

Fix: clean or rebuild the carburetor with a new float needle and seat, check the float height, and fix or replace the petcock. Our guide on how to clean a motorcycle carburetor covers the job. Turn the petcock off when parking a carbureted bike with a manual petcock.

Rich running and incomplete combustion

When there's more fuel than can burn, or a cylinder misfires, unburned fuel gets past the rings and into the oil. Causes include a choke left on or stuck, a dirty air filter, carburetors jetted too rich, a leaking injector, sensor faults on fuel-injected bikes, and weak spark. See our guide to motorcycle running rich.

Short trips, cold running and long idling

Cold engines leave more unburned fuel on the cylinder walls, some of which ends up in the oil. Normally, that fuel evaporates once the oil gets hot on a decent ride. Frequent short trips, long idling and stop-and-go riding can keep the oil from warming enough to evaporate the fuel, so it accumulates. Mixing in longer rides that bring the engine fully up to temperature helps.

Worn rings

Worn piston rings let more combustion gas and fuel into the crankcase. This usually comes with oil burning, blow-by and low compression. A compression or leak-down test confirms it.

How to check

Smell the oil on the dipstick or at the filler.
Compare the oil level to when it was last filled. Use the manual's checking method.
Look for fuel dripping from the carb overflow tube, air box or intake after the bike sits.
Check the petcock and vacuum hose. With the engine off, a vacuum petcock shouldn't flow fuel in "on."
Pull the plugs. A wet, fuel-soaked plug points to that cylinder.
Oil analysis. A lab test of a used oil sample can measure fuel dilution if you're unsure.

How to fix gas in motorcycle oil

Don't start the engine if you suspect a lot of fuel in the cylinders or crankcase. Pull the plugs and turn the engine over to clear fuel first, following your manual.
Change the oil and filter. Diluted oil should come out. See how to change motorcycle oil.
Fix the source: float needle, petcock, choke, injector, mixture or ignition.
Ride a longer warm-up ride and recheck the oil level and smell.
Recheck after a few hundred miles.

Typical costs

Costs vary by bike, region and shop.

Job	Rough range
Oil and filter change	About $60 to $200
Carburetor rebuild (single, with float needle)	About $100 to $300
Multi-carb rebuild and sync	About $300 to $700 or more
Petcock rebuild or replacement	About $40 to $250
Injector replacement	About $100 to $500 or more
Compression test	About $50 to $150

Prevention

Most fuel dilution is avoidable:

Turn off a manual petcock whenever you park a carbureted bike, especially for more than a day.
Don't leave a petcock in "prime" after starting.
Use fresh fuel and a stabilizer if the bike will sit, since varnish is what sticks float needles.
Clean or rebuild carburetors after long storage before problems start.
Ride long enough for the oil to reach full temperature, not just short hops around the block.
Avoid long idling at warm-up and in traffic.
Turn off the choke as soon as the engine will run without it.
Fix misfires and rich running quickly rather than living with them.
Check the oil level between changes. A level that creeps up is your earliest warning, long before you notice wear or poor running.
Change oil on schedule. Short-trip riding counts as severe service in many manuals, which calls for shorter oil change intervals.

Fuel-injected bikes

Fuel dilution is less common on fuel-injected bikes because there's no float bowl to overflow, and the injectors are shut off when the engine stops. When it does happen, look at a leaking injector that drips fuel after shutdown, a stuck-rich sensor reading (often a temperature sensor telling the computer the engine is cold), or a bike used mostly for short trips. A shop with a scan tool can check sensor readings and injector behavior.

When to see a mechanic

See a pro if:

The oil level is rising and you can't find why
Fuel is leaking from the carbs, petcock or fuel lines
The engine won't turn over after sitting (possible hydrolock)
The bike runs rough, smokes or keeps fouling plugs
You suspect worn rings or a leaking injector

A motorcycle diagnostics shop can test fuel delivery and ignition. Browse shops by state in our motorcycle repair directory.