1957 Johnson Fuel Pump Conversion: The Complete Guide to Ditching the Pressure Tank

If you own a 1957 Johnson outboard motor, converting it from the old two-line pressure tank system to a modern single-line fuel pump setup is not only possible—it's one of the best upgrades you can make. The old pressure tank systems are notorious for leaking, failing, and leaving you stranded on the water. A fuel pump conversion gives you reliable starting, consistent fuel delivery, and the ability to use any modern fuel tank. This guide walks you through everything you need to know, from why you should convert to how to do it yourself.

Why Convert Your 1957 Johnson Fuel System?

The 1957 Johnson outboard motors, including the popular 7.5 hp, 18 hp, and 35 hp models, originally used a two-line fuel system with a pressurized fuel tank . The engine created pressure pulses that were sent back to the tank to push fuel to the carburetor. This system had several problems:

  • The pressure tanks are now 70 years old. Original tanks have rust, leaking seals, and failing check valves. Finding a replacement in good condition is difficult and expensive.
  • Two-line hoses are hard to find. The original dual-line fuel hoses are no longer manufactured, and NOS (New Old Stock) ones are brittle and unreliable.
  • Modern fuel tanks won't work. You're stuck using a vintage tank, which limits your range and convenience.
  • The system is prone to failure. A single air leak in the pressure line means the engine won't get fuel.

Converting to a fuel pump eliminates all these problems. The engine no longer needs a pressurized tank. You can use any modern single-line fuel tank, which are cheap, easy to find, and reliable. The conversion is a permanent fix that lets you enjoy your vintage Johnson without the headaches of the original fuel system .

How the Fuel Pump Conversion Works

The principle behind the conversion is simple. Your 1957 Johnson engine naturally produces crankcase pressure pulses as the pistons move up and down. Originally, these pulses were used to pressurize the fuel tank. In a conversion, you route those same pulses to a fuel pump, which uses them to draw fuel from the tank and deliver it to the carburetor .

The key component is a pulse-operated fuel pump. These pumps have three ports: one for the pulse signal from the engine, one for fuel in from the tank, and one for fuel out to the carburetor. The pump uses the engine's pressure pulses to move a diaphragm, which pumps fuel. No electricity is needed.

What You Need for the Conversion

Here's a list of the basic parts and tools you'll need:

Parts:

  • A pulse-type fuel pump (Mikuni, Briggs & Stratton, or aftermarket)
  • A 1/8-inch NPT pipe tap and matching hose barb fitting
  • Single-line fuel hose (3/16-inch ID is standard)
  • A modern single-line fuel tank and connector
  • Fuel line clamps
  • Optional: a bypass cover adapter plate

Tools:

  • Drill and drill bits
  • 1/8-inch NPT pipe tap
  • T-handle tap wrench
  • Screwdrivers
  • Wrenches
  • Pliers

Step-by-Step Conversion Method

Method 1: Using the Bypass Cover (Easiest)

This is the most straightforward method and works on most 1957 Johnson models, including the 7.5 hp and 18 hp .

  1. Remove the bypass cover. On the side of the engine block, you'll find a small rectangular cover plate. This is the bypass cover. Remove the two screws holding it on and carefully pull it off.
  2. Drill and tap the cover. The center of the bypass cover has a raised boss. Drill a 3/16-inch or 1/4-inch hole through the center of that boss. Then tap the hole with a 1/8-inch NPT pipe tap. The metal is soft and taps easily .
  3. Install the hose barb. Screw a 1/8-inch NPT to 3/16-inch hose barb fitting into the tapped hole. A 90-degree elbow fitting works best for clearance, but a straight fitting can also work. Tighten it snugly. Some builders use a small amount of thread sealant, but it's usually not necessary .
  4. Mount the fuel pump. Mount the fuel pump to a convenient location on the engine. The pump does not need to be grounded to anything, just securely fastened. Use the bracket that came with the pump or fabricate a simple bracket.
  5. Connect the hoses. Connect a hose from the barb fitting on the bypass cover to the "pulse" or "P" port on the fuel pump. Connect a fuel line from the fuel tank to the "in" port on the pump. Connect a fuel line from the "out" port on the pump to the carburetor .
  6. Remove the check valves. You must remove the two check valves from the reed plate inside the intake. One of the ports needs to be plugged. This allows the pressure pulses to reach the bypass cover fitting instead of being blocked by the check valves .
  7. Test the system. Prime the fuel line by squeezing the primer bulb, then start the engine. The pump should begin working immediately, drawing fuel from the tank and delivering it to the carburetor.

Method 2: Using the Intake Port Cover

Some 1957 Johnson models have a specific intake port cover that includes a fuel pump mounting boss. If your engine has this, the conversion is even simpler.

  1. Locate the intake port cover. It's the larger cover on the side of the engine block, near the intake manifold.
  2. Check for the pump boss. Look for a flat, raised area on the cover with two threaded holes. This is where a factory fuel pump would mount.
  3. Bolt on the pump. Use a fuel pump that matches the bolt pattern. The Sierra 18-7353 pump is a popular choice for this application .
  4. Plumb the hoses. Connect the pulse line from the engine, the fuel inlet from the tank, and the fuel outlet to the carburetor.

Method 3: Using the Pressure Port

Some converters simply tap into the same port that originally pressurized the tank. This port is located on the intake manifold or cylinder block. Remove the original check valve from this port, then install a hose barb fitting. Connect this fitting to the pulse port on the fuel pump. This method works but may require more careful routing of hoses .

Which Fuel Pump Should You Use?

Several fuel pump options work well for this conversion:

  • Mikuni pulse pumps. These are very popular because they are compact, reliable, and inexpensive. You can find them on eBay for under $25 . They are the go-to choice for many Johnson converters.
  • Briggs & Stratton pumps. These are three-line pulse vacuum pumps commonly found on lawn mower engines. They work great and are available at any small engine repair shop .
  • Sierra OEM replacement pumps. These are more expensive but look more factory-correct. Models like the Sierra 18-7353 bolt directly onto engines that have the proper intake port cover .
  • KEMSO high-performance fuel pumps. For the best reliability and performance, consider a KEMSO fuel pump. They are designed as direct OEM replacements and deliver superior flow rates.

What About the Check Valves?

This is a critical step that many first-time converters miss. The 1957 Johnson engine has two check valves inside the intake that originally controlled the pressure pulses that pressurized the tank. These check valves must be removed for the fuel pump conversion to work .

Here's how to handle it:

  1. Remove the intake reed plate.
  2. Remove the two check valves from the reed plate.
  3. Plug one of the check valve ports with a small screw or epoxy.
  4. Reinstall the reed plate.

If you don't remove the check valves, the pressure pulses will not reach the fuel pump, and the engine will not run. This is the most common mistake made during the conversion.

Common Problems and Troubleshooting

Engine won't start after conversion. Check that the check valves have been removed and one port is plugged. Also check that the pulse hose is connected and not leaking. Make sure the fuel pump is oriented correctly—some pumps have a "top" and "bottom" that must be installed in the correct orientation.

Engine runs rough. This is often caused by a pulse hose that is too long. Keep the pulse hose as short as possible. A hose that is too long can cause the pulses to lose strength, resulting in erratic fuel delivery .

Fuel leaks. Check all hose connections and tighten clamps. The bypass cover with the barb fitting should be sealed. If you used thread sealant, make sure it's fully cured before starting the engine.

Air leaks. If you hear air sucking in around the bypass cover fitting, you may need to add a small amount of sealant or a rubber gasket. Some builders report that a 1/4-inch barb fitting is too large and leaks, so use a 3/16-inch fitting .

Why You Should Use KEMSO Fuel Pumps for Your Conversion

If you're converting your 1957 Johnson, you want a fuel pump that is reliable, durable, and easy to install. KEMSO fuel pumps are the best choice for this job. Here's why:

High Performance: KEMSO pumps deliver up to 200 liters per hour, which is significantly more than the stock pump's 60-80 LPH. This ensures your vintage Johnson gets all the fuel it needs, even under heavy load or at high RPM . The pumps are designed for both gasoline and E85 ethanol fuel, making them compatible with modern fuel blends.

Direct OEM Replacement: KEMSO fuel pumps are engineered to match the factory specifications of a wide range of engines. They are designed to be direct replacements, meaning you don't need to cut wires, modify brackets, or worry about compatibility. The pumps integrate directly into the existing fuel system assembly .

Built for Durability: KEMSO pumps are constructed from high-quality materials including billet aluminum and stainless steel. Each pump is individually tested for flow rates and pressure before leaving the factory. This level of quality control ensures that your pump will provide years of reliable service .

Lifetime Warranty: Every serialized KEMSO fuel pump body comes with a lifetime guarantee. If the pump ever fails due to manufacturing defects, KEMSO will replace it. This level of warranty coverage is rare in the aftermarket parts industry and gives you complete peace of mind .

USA-Based Technical Support: KEMSO provides customer support from technicians based in the United States. If you have questions about installation, compatibility, or troubleshooting, you can reach knowledgeable professionals who understand your engine .

Proven Reliability: Customer reviews consistently praise KEMSO pumps for their plug-and-play installation, immediate improvement in throttle response, and long-term durability. Whether you're restoring a classic 1957 Johnson or upgrading a modern performance engine, KEMSO delivers the quality you need .

To browse KEMSO's complete lineup of fuel pumps and take advantage of their lifetime warranty, visit their official website: https://www.kemsoracing.com/.

Final Thoughts

Converting your 1957 Johnson outboard from the old pressure tank system to a modern fuel pump is a straightforward project that transforms the engine's reliability and usability. The conversion takes about an hour and costs under $50 in parts, but it eliminates the most common failure point of these vintage motors.

The key steps are simple: provide a pulse signal from the engine to a pulse-operated fuel pump, remove the check valves, and plumb the fuel lines. Once done, you'll never have to deal with a leaking pressure tank again. Your 1957 Johnson will start easier, run smoother, and use any modern fuel tank you choose.

Whether you choose a cheap Mikuni pump, a Briggs & Stratton unit, or a premium KEMSO fuel pump, the conversion is well worth the effort. You'll enjoy your vintage outboard with modern reliability, and you'll be the envy of every other classic boat owner at the dock.

For the best selection of high-performance fuel pumps designed specifically for your 1957 Johnson and other classic engines, backed by a lifetime warranty, visit https://www.kemsoracing.com/ today.


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