A Primer on Primers

Proper Installation of Primer Bulbs in Fuel Lines

This article discusses the proper installation and operation of the primer bulb in fuel lines used with outboard motors.

Purpose of the Primer

Unlike your automobile engine, which typically has an electrical fuel pump located in the fuel tank itself that delivers fuel under pressure to the engine, most outboard motors have only a simple diaphragm fuel pump that operates from engine vacuum. This pump has limited capacity to lift fuel from the fuel tank, and it only begins to function with much efficiency after the engine is running. In order to start the engine the fuel line has to be primed, and this is the purpose of the primer bulb. If its fuel line is not primed, even an engine that is in perfect tune will will require a great deal of cranking to start. Rather than wearing down the battery to prime the fuel line, a small primer bulb pump is almost universally provided.

The primer bulb would seem like a simple device that does not require a great deal of explanation, but there are several details of its installation and use that may be overlooked.


Almost all primer bulbs consist of a flexible rubber bladder or bulb with inlet and outlet connectors. One-way check valves are located on the inlet and outlet side of the primer bulb. A slight spring pressure keeps the check valves closed, but not enough pressure to prevent the engine fuel pump from drawing fuel past it once the engine is running. Thus fuel can flow easily through the primer bulb in only one direction. Primer bulbs are almost universally marked with an arrow or some other identifier to indicate the direction of fuel flow through them. The inlet and outlet fittings typically contain a hose barb and these are generally available in three sizes to fit commonly used fuel lines. (Lower horsepower installations use 1/4-inch or 5/16-inch ID fuel lines; higher horsepower engines require 3/8-inch ID fuel lines. Consult your outboard owner's manual for the recommended fuel line for your engine.) Use a primer bulb whose connections are sized to match your fuel lines.

When the primer bulb is squeezed, the pressure created closes the inlet valve and opens the outlet valve, allowing fuel in the bulb to flow toward the engine. When the bulb is released, the suction created closes the outlet valve and opens the inlet valve, drawing fuel from the tank into the bulb. Repeated squeezing causes fuel to be lifted from the tank and pumped to the engine, until the fuel line is filled and the bulb becomes firm.

When the engine is running its fuel pump produces a suction that draws fuel past the check valves. The valves are designed so that even the modest pressure of the engine fuel pump will draw fuel past them. At idle speeds some engine fuel pumps may produce marginal pressure and may not draw fuel past the check valves. This can lead to stalling and other problems. OEM brands of primer bulbs have been designed to work with the fuel pumps in their engines, and their use is highly recommended over after-market products. Recently Mercury Marine came out with an improved primer bulb and is recommending it for all their engines. It can be identified by the yellow plastic line visible where the fittings join the rubber bulb.

Another important consideration is the flexibility of the rubber components. In the past decade it has become very common for gasoline fuels to be blended with ethanol. Unfortunately, ethanol tends to attack rubber and to reduce its flexibility. Any rubber components in the fuel system must now be made from alcohol-resistant rubber compounds. Older rubber hoses, primer bulbs, gaskets, etc., should be replaced.


The primer bulb is typically installed in the flexible rubber fuel hose feeding the engine. The fuel hoses are pressed onto the inlet and outlet fittings and retained with clamps. It goes without saying the engine fuel system must be leak proof, but it is also necessary that it be air-tight as well. Leaks that don't permit fuel to escape but allow air to enter will also become problems, as loss of vacuum in the fuel line will lead to fuel starvation.

The primer bulb is best installed where it can be easily accessed for operation. In most pre-rigged boats being delivered these days, the primer bulb is seen located within 12-18 inches of the engine. This is especially common on boats that have internal fuel tanks. Boats with on-deck fuel tanks may have the primer bulb located at the tank end of the fuel line. Of course, the primer bulb must be installed in the fuel line with the fuel flow direction arrow oriented properly, pointing toward the engine.

The primer bulb is generally placed close to the engine, and usually downstream of any fuel filters or water-separtors. Many new engines are delivered without a fuel disconnect fitting, and instead just provide a short stub of fuel line from the engine cowling. This makes a natural place to connect a primer bulb.


It is also advantageous if the primer bulb can be position in such a way that during priming it can be oriented vertically, with the direction arrow pointing skywards. This will allow gravity and the weight of the fuel inside the primer bulb to help with the operation of the one-way valves. If the primer bulb is oriented with the flow arrow pointing downward, gravity and the weight of the fuel in the line above the inlet check valve may spoil its operation, and the primer will not work properly.

This subtle but important point—orienting the primer bulb skyward—is often overlooked in many installation, but it really helps the primer bulb to do its job. This trick was shown to me by an experienced Mercury outboard mechanic, after I complained that the new primer bulbs he had installed did not work well. The difference in results is amazing, and just by changing the orientation of the primer to vertical, its operation is much improved. In just a few squeezes it should be possible to fill the bulb with fuel.


Photo: 2004 Boston Whaler 305 CONQUEST with Optimax 250 engine showing primer bulb installation
Primer Bulb
In this factory-rigged installation the primer bulb is located close to the engine. It is oriented vertically, too, so that it primes easily.
PhotoCredit: James W. Hebert


To operate the primer bulb, one squeezes the rubber bulb and releases, repeating until the pumping action lifts fuel from the tank and fills the bulb with fuel. As the bulb fills its resistance to squeezing changes, and the firmness of the bulb signals the operator when the primer has done its job and primed the fuel line.

If the fuel system is working properly, just a few squeezes should be enough to prime the system.

Primer Bulb As Diagnostic Tool

The primer bulb is also a diagnostic tool that will tell you a number of things about your fuel system.

Engine Stalling or Running Roughly at Speed

If your engine begins to stall or run roughly several minutes after starting and when running at higher speeds, use the primer bulb to force fuel to the engine. If this restores the engine to smooth running, you have a fuel supply problem. There may be a restriction in the fuel line, or your engine fuel pump could be defective. If this happens even at idle speeds, the primer bulb may not be working properly.

Primer Hard to Prime

If the fuel line is difficult to prime, you likely have an air leak in the system. Check all fuel hose connections. You could also have a defective primer bulb, or not be orientating the primer bulb properly during priming. A likely place to investigate is the fuel disconnect on the engine. Be sure the connector is properly seated and is making an air-tight connection.

Collapsed Primer Bulb

If the primer bulb collapses after the engine has been running or stays collapsed while priming this indicates a fuel line restriction upstream from the primer bulb. A defective check valve in the primer bulb could also be the cause. The fuel line filter or the fuel tank pick up could be blocked. A blocked fuel tank vent could also cause this. A quick way to check the tank venting is to temporarily open the fuel tank filler fitting. For some reason the fuel tank vent line thru-hull fitting is a favorite spot for wasps to build mud nests.

Primer Bulb Never Gets Firm

If you can fill the bulb with fuel but it never gets firm, you probably have a fuel leak downstream of the primer bulb. This could be a stuck float valve on a carburetor, or a fuel leak in the engine fuel pump. Many engines use a diaphragm pump, and if the rubber diaphragm has a pin hole leak, gasoline will be forced past it into the engine crankcase. It should only take a few squeezes to get a primer bulb to become firm.

If you have a fuel disconnect downstream of the primer bulb, disconnect the fuel line from the engine and see if the primer can now be pumped to firmness. If so, your leak is in the engine.

Primer Bulb Gets Soft While Engine Is Running

Most primer bulbs will lose some firmness when the engine is running. This is normal. The bulb should not run dry of fuel or collapse in suction, but it will tend to lose some of the hard firmness it had when initially primed.

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!

The page has been accessed times.

Portions Copyright © 2003 by James W. Hebert. Unauthorized reproduction prohibited!

This is a verified HTML 4.0 document served to you from continuousWave
URI: http://continuouswave.com
Last modified:
Author:James W. Hebert
This article first appeared February 22, 2004.