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 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.

Leaking Carburetor Needle

Your ATV carburetor is a multi tasking precision piece of kit. It’s got 3 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 insider the air-box.

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 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 gas level rises the needle is once again seated and 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, this as you can imagine allows gas seep into the bowl.

The gas fills the carburetor and very often overflows into 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.

Hydro-locking is also common, when gas fills the cylinder, it can’t be compressed and so the piston simply won’t move.

Hitting the start button offers a click of the starter and nothing else, the rider understandably assumes its a flat battery.

Checking the Needle

A leaking needle and seat is 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.

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

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

Fuel Float

The fuel float lives inside the bowl and controls 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 possible the carburetor. The float may have a metal or plastic tang, the metal are adjustable but the plastic aren’t.

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

A float with a plastic tang will need to be replaced.

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 employ a rubber diaphragm and a vacuum line from the carburetor. As the engine cranks, engine vacuum pulls on the diaphragm and actuates the valve.

Problems with older diaphragms can occur and may allow gas 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 by pass the valve and block the vacuum line for the purposes of 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 contaminates inside the cylinder head, leading to overheating, pre-ignition, burnt valves and seats.

After you find and fix the cause of your your leak, it’s very important to change your oil, oil filter, spark plug and clean the air filter.

John Cunningham

John Cunningham is an certified mechanic and writer on I’ve been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance, repair to troubleshooting.

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