How to Diagnose and Replace an Outboard Primer Bulb

Squeeze the bulb with the engine off, then watch it while the engine runs (don't squeeze it while running; see step 10). What it does tells you where to look:

• Won't firm up no matter how much you squeeze: air is getting in somewhere (a connector, its O-ring, a split hose, a loose clamp), the bulb's valves are bad, or the tank is nearly empty or its pickup is cracked.
• Firms up, then goes soft quickly: a leak downstream (connector at the engine, its O-ring, or fuel bleeding past a worn valve).
• Collapses flat while the engine runs: the engine is pulling against a restriction: a closed or blocked tank vent, a kinked line, a clogged filter or tank pickup screen. Don't forget the short hoses between the filter and the engine: owners on older motors have found an inner hose lining that had broken down and collapsed, even though the outside looked fine.
• Hard and stiff, hard to squeeze: old rubber and failing valves (BoatUS).
• Wet, swollen, cracked or smells of gas: replace it now.

Most "collapsing bulb" problems are the tank, not the bulb. On an older portable tank, open the vent screw on the cap. Newer EPA-style portable tanks are sealed and use a built-in valve to manage pressure and vacuum instead of a manual vent; if one of those pulls a strong vacuum (the cap hisses loudly when opened, or the tank sides suck in), the valve or cap may be faulty. Run with the cap loosened briefly as a test only, in open air, then fix or replace the cap or tank. See our fuel tank inspection guide.

Unplug the line at the tank and the engine. Look inside each quick connector at its small O-ring: hard, flattened, cracked or missing O-rings let air in and fuel out, and a new bulb won't fix that. Make sure each connector clicks and locks firmly. If a connector itself is cracked or loose on its barb, replace it with one made for your engine brand.

Buy a marine, EPA-compliant, ethanol-rated bulb with barbs that match your hose. Get all-stainless hose clamps sized for the hose, or use the clamps supplied with the bulb as its instructions say. Bring the old bulb or a piece of hose to the store if you're unsure.

Engine off, key out or kill-switch lanyard removed. Loosen the portable tank cap slowly, away from the engine, to release any pressure, then retighten it. Disconnect the fuel line from the engine and the tank.

Hold the line over an approved gasoline container with the tank end down and squeeze the bulb to push fuel out. Catch drips with an absorbent pad. Never siphon fuel with your mouth; BoatUS calls that "unbelievably dangerous."

Note or photograph which way the old arrow pointed. Remove both clamps (cut crimp clamps carefully with side cutters, without nicking the hose) and twist the hose off each barb. Check the last inch or two of hose: if it's hard, cracked or split from the barb, trim it square with a sharp knife, but only if there's enough slack left for the engine to turn and tilt. If the hose is generally hard or cracked, stop here and replace the whole line.

Find the arrow on the new bulb and point it toward the engine. Push the hose fully onto each barb, all the way to the shoulder. Don't use grease, soap or oil to help it slide; they can let the hose work loose or contaminate the fuel. Keep the hose straight onto the barb so it doesn't kink at the joint.

Put a clamp on each end of the bulb, just behind the barb's ridge. Tighten screw clamps snug by hand with a nut driver or screwdriver, not a power tool, so you don't cut the hose. If your bulb came with crimp or snap clamps, install them as the instructions show.

Reconnect the line to the tank and engine. Squeeze the bulb until it becomes firm. Then:

• Wrap a dry paper towel around each bulb end and each connector and look for wet spots.
• Sniff along the line.
• Leave it firm for a few minutes: it should stay firm.

After priming, stop squeezing. Several outboard makers' owner's manuals warn against squeezing the bulb while the engine is running or tilted up, because fuel can overflow or flood the engine. Check your manual for your model's priming instructions (some tell you how to hold the bulb while priming).

Start the engine in the water, in a test tank, or on external flush muffs with the water running first (never on the built-in flush port with the engine running unless your manual allows it). Let it idle a few minutes. Watch the bulb without touching it: it should stay roughly round. If it slowly collapses, go back to step 2. Shut down and recheck every joint with a dry paper towel.

Make a short run at different speeds, then check the joints again. Take drained fuel to a household hazardous waste site. Let the old bulb and any trimmed hose air out outdoors until dry, then dispose of them according to your local rules.