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ATV Oil Smells Like Gas (Fixed)

Smelling raw gas from your ATV can be alarming, especially if you park it in the garage hot.

A smell of gas from ATV oil is commonly caused by:

  1. Leaking carburetor valve needle or seat
  2. Incorrect fuel float setting
  3. Faulty petcock

In this post, you’ll learn why your ATV oil smells like gas, how you can diagnose the problem, and how you can fix it. We’ll also cover the dangers of contaminated crankcase oil.

1 Leaking Carburetor Float Valve

Your ATV carburetor is a multi-tasking precision piece of kit. It’s got three main jobs:

  1. Mix air and gas and to a precise ratio
  2. Supply the correct quantity for any given engine load
  3. Keep a sufficient quantity in the fuel bowl at all times

When I smell raw gas from engine oil, I immediately suspect a possible problem with the carburetor float needle. A common symptom is, as you know, the smell of gas but also an unusually high oil level and possibly gas inside the air-box.

ATV carburetor needle valve

Your carburetor is fitted with a fuel bowl. It’s a reserve of gas that stands ready to supply the engine. The gas level in the bowl is controlled by the float, float needle, and seat.

The needle and seat block a fuel passage, and attached to the needle is the float which rises and falls with the gas level inside the bowl.

As the float falls, the needle unseats, and gas fills the bowl. As the gas level rises, the needle is once again seated, and the gas flow stops. A rubber seal is used either on the tip of the needle or in the seat. As the needle gets older, it perishes and splits, as you can imagine allowing gas to seep into the bowl.

The gas fills the carburetor and very often overflows into the cylinder through an open intake valve. Gravity forces the gas inside the cylinder to leak past the rings and make its way to the crankcase.

ATV cylinder water

Hydro-locking is also common when bikes get stuck in water, but gas can also cause hydro-locking. When gas fills the cylinder, it can’t be compressed, so the piston won’t move.

Hitting the start button offers a click of the starter and nothing else. The rider understandably assumes it’s a flat battery.

Checking the Needle

A leaking needle and seat are so common that if you own your bike long enough, it will happen. To confirm a leaking needle, remove the fuel bowl. Some bikes won’t make this easy. You may need to remove the whole carburetor.

Carburetor location on dirt bike

With the bowl removed and a fuel line connected, lift the float. Fuel should stop flowing. Examine the needle seal. It may be on the tip or in the seat.

If your carb model has a needle and seat, go ahead and replace both, nobody likes visiting the same job twice.

2 Fuel Float

The fuel float lives inside the bowl and controls the fuel bowl fuel level. If the float isn’t set correctly, it may overfill the bowl, which will have the same effect as a leaking needle. Setting the bowl will require removing the fuel bowl and possibly the carburetor. The float may have a metal or plastic tang, the metal is adjustable, but the plastic isn’t.

To adjust the metal tang, first, you’ll find your carb float specs. Then you’ll need to measure float travel and adjust the metal tang by simply bending it and rechecking.

ATV float adjusting

A worn float with a plastic tang will need to be replaced as it can’t be adjusted.

3 Petcock Faulty

A petcock or fuel valve is fitted to your fuel line. Two types are common, the manual type and the vacuum-operated auto type. The vacuum-operated type employs a rubber diaphragm and a vacuum line from the carburetor. As the engine cranks, the engine vacuum pulls on the diaphragm and actuates the valve.

Problems with older diaphragms can occur and may allow gas to leak past the diaphragm and into the combustion chamber. Your engine will run extremely rich. You may notice black smoke, backfiring, and bogging. A rich running engine will cause misfiring, and that means un-burnt gas will make its way to the crankcase.

To check the petcock, remove the vacuum line. Gas in the line spells a failed diaphragm. You can also bypass the valve and block the vacuum line for testing.

Dangers of Contaminated Oil

The dangers associated with a rich running ATV engine are real, gas in the oil dilutes it, and that means your internal engine components aren’t lubed or cooled correctly. In addition, raw gas washing down the cylinder removes the protective oil coating. This means premature engine wear or total failure.

A rich running engine will also allow a build-up of contaminants inside the cylinder head, leading to overheating, pre-ignition, burnt valves, and seats. After you find and fix the cause of your leak, it’s very important to change your oil, oil filter, spark plug, and clean the air filter.

You’ll find the tools I recommend here on the ATV tools page and you’ll find common spare parts here on the ATV parts page.

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