How to Keep a Prime on an Outboard Motor

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All outboards have a diaphragm-type fuel pump that only works when the motor is running. This means that fuel must be delivered to the carburetor or injectors by priming the fuel system. Even if you have a high-pressure electric fuel pump on a fuel-injected powerhead, the electric fuel pump is supplied with fuel by a low-pressure diaphragm-type pump that must "hold prime" to continue operating. If it does not, the motor loses prime and ceases to run.

Items you will need

  • Clear plastic A1 fuel line

  • Clean fuel

  • 5/16-inch box-end wrench

Step 1

Open the cap on the fuel tank before trying to start the motor. Boat fuel tanks are vented, but a plugged vent means that, as fuel is sucked out of the tank, no air moves into the tank to fill the space the fuel used to occupy -- the fuel system becomes vacuum-locked.

Step 2

Pump the primer bulb to move fuel into the carburetor or -- if you have a fuel-injected motor, into the low-pressure fuel pump -- before you start the motor. Pump the bulb until it is firm to the touch.

Step 3

Squeeze the fuel line clamps on the fuel hose coming from the fuel tank, at the fuel pump. Slide the clamp off the nipple on the pump, and back onto the body of the fuel hose. Push the fuel line off the carburetor nipple.

Step 4

Push the end of a clear plastic fuel line onto the pump nipple. Slide the clamp of the clear plastic line onto the pump nipple.

Step 5

Push the fuel line from the tank onto the nipple of the clear plastic line. Slide the clamp of the line from the tank line onto the clear plastic line's nipple.

Step 6

Pressurize the system with the primer bulb. Visually inspect the fuel system for leaks.

Step 7

Buy good quality fuel only. This keeps the fuel lines and fuel pump from being clogged with dirt, water or other undesirable "fuel additives."


  • Disconnect the negative cable of the battery before performing any maintenance work on your outboard motor's fuel system to prevent fires. Remove the nut from the negative post with a 5/16-inch box-end wrench. Lift the cable from the battery, move it outside of the battery box and close the lid of the battery box. After the work is complete, reconnect the negative battery cable.


  • Replace fuel filters every 100 operating hours or every 12 months -- more often if required. If you run your motor less than 100 hours in a season, replace the fuel filters as you prepare your boat for the boat- season.
  • Clear plastic fuel lines meeting the U.S. Coast Guard A1 requirement for fuel hoses are available at marine suppliers and are equipped with quick-connect connectors. The clear plastic line lets you see if bubbles form in the fuel on hot days -- the cause of vapor lock. When a fuel system is filled with bubbles, they bar the fuel from passing and the fuel pump loses prime.
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