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Gravity Siphon Method

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Siphoning Techniques
Safe extraction from tanks and containers
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Gravity Siphon Method
Hose, jiggle, and flow — the basic technique

A gravity siphon works only while the hose is full of fuel and its outlet sits below the fuel surface in the tank, so prime the hose with a squeeze bulb or by filling it before use, never by mouth. Keep the intake end submerged and a little off the tank floor, keep the container on the ground, and stop the flow by lifting the outlet above the fuel level or pinching the hose before the container is full.

Part of Fuel Siphoning & Storage

A gravity siphon needs only a hose and a container, which is why people reach for it first, and its only hard step is getting the hose full of fuel without a mouth.

Priming Is The Only Hard Step, And There Are Four Ways To Do It Without The Mouth

Once the hose is full of fuel and the outlet is lower than the fuel level in the tank, gravity keeps the fuel moving. Before that, the hose holds air, and something must replace the air with fuel. Never do it by mouth. Gasoline drawn into the lungs causes chemical pneumonitis, which can be fatal and has no field treatment, and the fuel reaches the mouth faster than people expect — it is the reflex cough at the taste that inhales it.

What You Need

  • Clear, fuel-rated flexible tubing, 1-2 cm bore and at least 2 m long (parent article).
  • A receiving container that can sit on the ground, lower than the tank.
  • A squeeze bulb, if you have one, or a second short length of hose for the air-push variant.
  • Rag, sand and a bucket for spills.

Method: Pick One Priming Route, Then Run The Flow

Step 1: Set up. Work outdoors, stand upwind and put the container on bare ground below the tank. Open the filler cap, with nothing burning or sparking nearby.

Step 2: Insert the intake. Feed the hose into the tank until it touches the bottom, then pull it back 2-3 cm so the end is not sealed against the floor (parent article). The end must stay below the fuel surface for the whole transfer, or air enters and the flow breaks.

Step 3: Prime, by one of these.

  • Squeeze bulb. Fit the bulb in the hose and squeeze until fuel appears in the clear tubing past the bulb. Then lower the outlet into the container.
  • Pre-filled hose. Submerge the whole hose in a container of clean fuel until no air bubbles rise, close both ends with your thumbs, put one end into the tank and release the lower end into the container. If no fuel is available, the parent article describes filling the hose with water, which then runs into your container ahead of the fuel and contaminates it; use only if you can spare a first splash.
  • Short-stroke method. Hold the free end lower than the tank and push the hose in and out of the tank by 15-20 cm, quickly. It takes 10-30 seconds of vigorous work (parent article), and splashes. Keep your face back.
  • Air push (two-hose). Pass the long intake hose and a short air hose through the filler, pack a rag loosely around them and give gentle puffs from a hand squeeze bulb. Stop the moment fuel flows. Never blow into the air hose by mouth, never use a compressor or engine exhaust, and keep the pressure gentle, because a plastic tank is not designed to be pressurised.

Step 4: Watch the flow. You should see a steady stream. If it stops, the intake may be out of the fuel, or air has entered; re-prime from Step 3.

Step 5: Stop before the container is full. Raise the outlet end above the fuel level in the tank, or pinch the hose, while there is still an air space at the top of the container. Never leave an unattended siphon running.

Step 6: Withdraw the hose with the end raised, to avoid dripping on the ground and the bodywork.

Common Mistakes

  • Letting the intake rest on the tank floor. The hose seals itself against the bottom and the flow stalls.
  • Forgetting to watch the container. Overflow is the commonest spill.
  • Reusing a hose that has held a different fuel. Residue contaminates the next one.

Safety

Never prime by mouth

Gasoline swallowed burns the throat and stomach, and gasoline breathed into the lungs causes chemical pneumonitis, which can be fatal. If fuel reaches your mouth, do not make yourself vomit; rinse and spit, and get medical help if you can. Keep every ignition source away, keep the container on the ground with the hose in contact with its rim to discharge static, and never siphon into a plastic container on a car seat or carpet.

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