The primer bulb is the rubber squeeze bulb in the fuel line between the tank and the outboard. Squeezing it pushes fuel to the engine before starting; it should become firm after a few squeezes. If it will not firm up, keeps going soft, or collapses while the engine runs, the problem is usually an air leak, a failed bulb or check valve, or a restriction between the tank and the bulb, such as a closed tank vent, a blocked pickup or a sticking anti-siphon valve.
Primer bulb problems and causes
| Symptom | Likely cause |
|---|---|
| Bulb never gets firm | Air leak at fittings, cracked bulb or hose, failed check valve |
| Bulb firms, then slowly goes soft | Fuel leaking back through a check valve, or a leak downstream |
| Bulb collapses while running | Restriction upstream: closed tank vent, anti-siphon valve, clogged pickup or filter |
| Fuel smell at the bulb | Cracked or perished bulb or hose |
Outboard primer bulb keeps going soft
When the bulb goes flat while the engine is running, the engine is pulling fuel harder than it can flow from the tank. Open the tank vent and see whether the bulb recovers. If not, check the anti-siphon valve at the tank outlet and the fuel filter; see outboard fuel filter. This often shows up as a loss of power at full throttle; see outboard loses power at full throttle.
Outboard primer bulb installation
- Fit the bulb with its arrow pointing toward the engine, the direction of fuel flow.
- Use ethanol-rated fuel line and new clamps; see ethanol in boat fuel.
- Keep the bulb where it is protected from sun and fuel spills, and avoid sharp bends in the hose.
- Prime and check every connection for leaks before starting.
Testing a primer bulb
Disconnect the line at the engine, point it into a container and squeeze the bulb: fuel should flow out in steady pulses. Then pinch the line on the engine side and squeeze; the bulb should firm up and stay firm. If it slowly softens with the outlet pinched, a check valve inside the bulb is leaking back.
When to replace it
Replace a primer bulb that is cracked, sticky, hard or that leaks fuel. Many owners replace the bulb and fuel line together every few seasons, especially with ethanol fuel.
Priming tips
- Squeeze until firm before starting, not repeatedly while the engine runs.
- If the engine starts and dies, check whether the bulb stays firm.
- Keep the tank vent open whenever the engine runs.
Related
See outboard won’t start, outboard won’t idle or stay running and outboard motor maintenance.
More guides: EFI vs carbureted · outboard fuel lines.
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