# Outboard Fuel Lines, Primer Bulbs and Filters: Finding the Air Leak **By Sawley's Auto and Marine** ยท 2026-10-04 Our [diesel fuel filter guide](/blogs/guides/diesel-fuel-filters-water-separators) dealt with the diesel side of the fuel system. Petrol outboards have their own set of problems, and the big one is air. A diesel with air in its fuel stalls and will not restart until it is bled; a petrol outboard with air in its fuel line runs, badly, and makes you chase the engine for a fault that is actually in a two dollar hose fitting. This guide follows the fuel from the tank to the engine and shows where the air gets in, how to find it with the primer bulb and your eyes, and what to replace while you are there. ## Follow the fuel Petrol leaves the tank through a pickup tube, passes an anti-siphon valve on a built-in tank, runs through the fuel line to a primer bulb, through a water-separating filter if one is fitted, then through a connector into the engine, where an inline filter and a fuel pump feed the carburettors or, on an EFI or direct-injection engine, a vapour separator tank and a high-pressure pump. Every joint in that path is a place air can be drawn in on the suction side, where the pressure is below atmospheric, without a drop of fuel leaking out. That is why a fuel leak is easy to find and an air leak is not. ## What the primer bulb tells you The primer bulb is a hand pump with a check valve at each end, and it is also the best diagnostic tool on the boat. Squeeze it before starting. It should firm up in a few pumps and stay firm. A bulb that never goes hard has air coming in somewhere between the tank and the bulb, a stuck check valve in the bulb itself, or a pickup that is not reaching the fuel. A bulb that goes hard and then softens over a minute has a leak-back, usually the bulb's outlet valve or a fuel connector O-ring. A bulb that collapses flat while the engine runs is a restriction upstream: a blocked tank vent, which you can prove by cracking the filler cap, a kinked line, a clogged pickup screen or a stuck anti-siphon valve. An engine that runs well for ten minutes then stumbles and dies, and runs again after the bulb is pumped, has a fuel pump that is not keeping up, which is usually a split diaphragm. ## Hose Marine fuel hose is not the stuff from the car accessory shop. It has to be rated for marine use, marked as such along its length, and on any boat built or re-fuelled since ethanol blends arrived it has to be rated for them, because ethanol softens and swells ordinary rubber from the inside. Old hose fails invisibly: the outside looks fine while the inner liner cracks, flakes into the filter, and lets air in through the wall. Squeeze it along its length; hard, glossy or cracked at the fittings means replace it, all of it, not the bad metre. Use the correct size for the engine, double-clamp at the tank end and use the right clamps, which is a subject on its own. Hose by the metre, primer bulbs and connectors are in the [marine fuel hose range](/collections/hose-fuel-marine-fuel-line-primer-bulb-fuel-hose), and a genuine [Quicksilver primer bulb](/products/8m0054313-mercury-mercruiser-13330q07-primer-bulb-genuine-quicksilver-oem-part) is worth the small premium over a generic one because the check valves are what you are paying for. ## Connectors and O-rings The quick connector at the engine and the one at a portable tank each seal on a small O-ring, and those O-rings harden with age and ethanol. A connector that needs wiggling, or that sits slightly off the barb, is drawing air at idle and leaking at rest. The SA boating handbook has O-rings for the fuel connector on its list of spares to carry, and so do we. On engines with a fuel line that pushes onto a barb and is held by a clamp, a barb that has been scored by a screwdriver during a past hose change will never seal properly again. Replace the barb or dress it smooth. ## Filters and water A water-separating filter between tank and engine is the one accessory every petrol outboard over a few horsepower should have. Water sinks to the bottom of the bowl where you can see it and drain it, rather than reaching the carburettor or the injectors. On the Peninsula the water comes from condensation in half-empty tanks over winter and from the fuel itself, and the [stale fuel guide](/blogs/guides/why-your-outboard-wont-start-in-spring) covers why ethanol makes that worse. The [Ryco range](/collections/ryco-filters-australia-oil-fuel-air-filters-for-automotive-marine) includes universal separator kits with a bowl and a ten micron element. Change the element at least yearly, and the small inline filter under the cowl at the same time. On a two-stroke that runs premix, a blocked filter leans the mixture as well as starving it, which is a seized piston rather than a stall. ## Tanks and vents A portable tank with a vent screw that has been left closed will stall the engine within minutes; a built-in tank with a vent line that is kinked, blocked by a mud wasp or submerged by a fitting that has been painted over does the same more slowly. The anti-siphon valve on a built-in tank, a spring-loaded ball at the pickup, sticks with varnish from old fuel and is a frequent cause of fuel starvation at speed. Fittings, pickups, valves and senders are in the [fuel system parts range](/collections/fuel-systems-fuel-pumps-lines-tanks-components). If you are chasing an outboard that will not idle or dies at full throttle, ring [0412 532 984](tel:+61412532984) and describe what the bulb does, because that is the first thing we will ask. **Tags:** Guides, Outboards, Servicing --- > Source: [Sawley's Auto and Marine](https://www.sawleysautoandmarine.com.au/blogs/guides/outboard-fuel-lines-primer-bulbs-filters-air-leak)