Johnson 70 HP Fuel Pump: The Complete Guide to Replacement, Troubleshooting, and Finding the Right Part
If you own a Johnson or Evinrude 70 HP outboard motor and are experiencing hard starting, sputtering at high RPM, or a complete failure to run, the fuel pump is almost always the first component to check. The most common replacement part for these engines is the 438559 fuel pump (also cross-referenced as 385784, 395712, 398385, and 433390), which fits Johnson and Evinrude 70 HP motors built between 1974 and 1983, as well as many other models ranging from 25 HP to 140 HP . This guide will walk you through how to identify a bad fuel pump, how to replace it, and where to get a reliable replacement that will last.
How to Tell If Your Johnson 70 HP Fuel Pump Is Failing
A failing fuel pump on a Johnson 70 HP outboard will show clear symptoms. You don't need to be a mechanic to spot them. Here are the most common signs:
1. Engine Won't Start or Starts Hard
The most obvious symptom is that your motor cranks but refuses to fire up. If the fuel pump is not delivering enough fuel to the carburetor, the engine simply cannot run. This is often the first sign of a pump that has lost its internal diaphragm or check valve .
2. Engine Sputters or Loses Power at High Speed
If your 70 HP Johnson runs fine at idle but starts to cough, sputter, or lose power when you open the throttle, the fuel pump is likely struggling to keep up with demand. The pump may be delivering enough fuel for low-speed operation but not enough for high-RPM running .
3. Fuel Leaks
Look for fuel dripping from the pump body or from the small pulse hose that connects to the engine block. A leaking diaphragm inside the pump will push fuel out of the pulse port. This is a safety hazard and needs immediate attention .
4. Oil in the Fuel Line or Pump
If you are running a premix oil-fuel system, and you see excessive oil pooling around the pump or in the fuel line, the internal diaphragm may be ruptured. This allows fuel to mix with the crankcase oil, which can cause serious engine damage .
Which Johnson 70 HP Models Use the 438559 Fuel Pump?
The 438559 fuel pump (and its equivalent part numbers 385784, 395712, 398385, and 433390) is a direct replacement for a wide range of Johnson and Evinrude outboards. For 70 HP motors specifically, it fits the following models:
- Johnson 70 HP models from 1974 to 1983 (including models like J70ELCNB, J70ERCNB, and similar)
- Evinrude 70 HP models from 1974 to 1983 (including models like E70ELCNB, E70ERCNB, and similar)
- Crossflow engines – This pump is designed for the Crossflow series of 2-stroke outboards, which includes most 70 HP motors from that era
It is important to note that the V6 engines (140 HP and above) require two of these pumps, but your single-cylinder or two-cylinder 70 HP engine only needs one .
How to Replace the Fuel Pump on a Johnson 70 HP
Replacing the fuel pump on a Johnson 70 HP outboard is a straightforward job that you can do in about 30 minutes. Here is the step-by-step process:
Step 1: Gather the Right Parts
You will need a replacement fuel pump that matches the 438559 specification. Most aftermarket kits come with the pump itself, a gasket, fuel filters, and clamps . Avoid using universal pumps that require cutting or modifying your fuel lines. A direct-fit replacement is always better.
Step 2: Disconnect the Battery
Before you start working on any fuel system component, disconnect the negative terminal of your battery to prevent any accidental sparks.
Step 3: Locate the Fuel Pump
The fuel pump is mounted on the side of the engine block, usually near the carburetor. It has three nipples: one for the fuel inlet (from the fuel tank), one for the fuel outlet (to the carburetor), and one for the pulse line (a small hose that connects to the engine crankcase) .
Step 4: Remove the Old Pump
Using a small screwdriver or a 10mm socket, remove the two bolts that hold the pump to the engine block. Carefully pull the pump away from the block. Be prepared for a small amount of fuel to spill.
Step 5: Clean the Mounting Surface
Use a clean rag to wipe the mounting surface on the engine block. Make sure there is no old gasket material or debris left behind.
Step 6: Install the New Pump
Place the new gasket onto the mounting surface, then position the new fuel pump. Secure it with the two bolts. Do not overtighten – just snug them up.
Step 7: Reconnect the Fuel Lines
Connect the fuel inlet line, the fuel outlet line, and the pulse line. Make sure all connections are tight and secure with the provided clamps .
Step 8: Test the System
Reconnect the battery, prime the fuel system (squeeze the primer bulb until it is firm), and start the engine. Let it run for a few minutes and check for any leaks. Take the boat out for a test run to ensure the engine runs smoothly at all RPMs.
Why You Should Choose a KEMSO Fuel Pump for Your Johnson 70 HP
When it comes to replacing the fuel pump on your Johnson 70 HP outboard, you have many options. However, not all aftermarket pumps are created equal. If you want a part that is built to last, performs better than the original, and comes with a lifetime warranty, KEMSO Fuel Pumps are the best choice for your motor.
KEMSO is a trusted brand in the United States, and their fuel pumps are designed as direct OEM replacements. This means you do not need to modify anything on your engine. The pump drops right into the factory location and works with your existing fuel lines. Here is why KEMSO stands out from the competition:
- High Performance: KEMSO fuel pumps deliver up to 200 liters per hour at 43 psi, which is 20-30% more flow than many stock pumps. This extra flow ensures your engine gets the fuel it needs, even at high RPMs .
- OEM Replacement Fit: Each KEMSO pump is vehicle-specific. You get a pump that is designed to match the exact dimensions and specifications of your Johnson 70 HP motor .
- Lifetime Warranty: KEMSO backs every pump with a lifetime warranty. The pump body is serialized for tracking, so you never have to worry about buying a replacement again. You buy it once, and KEMSO stands behind it .
- USA-Based Support: If you have any questions during installation or need help diagnosing a problem, KEMSO's technical support team is based in the USA and ready to help .
- Ethanol Compatible: KEMSO pumps are fully compatible with both gasoline and E85/ethanol fuels. This is critical for anyone who uses ethanol-blended fuel at the pump .
- Quiet and Reliable: Users consistently report that KEMSO pumps are quiet and easy to install. Independent lab tests show KEMSO pumps maintain 98% efficiency even after 15,000 hours of continuous operation .
- Competitive Pricing: At a regular price of just $79.98, KEMSO fuel pumps are significantly cheaper than OEM replacements and many premium aftermarket brands .
Visit the KEMSO website today to find the perfect fuel pump for your Johnson 70 HP outboard. You can browse their full selection of high-performance and OEM replacement pumps at:
Final Thoughts
A failing fuel pump on your Johnson 70 HP outboard does not have to ruin your day on the water. By recognizing the symptoms early and replacing the pump with a quality part, you can get back to fishing, cruising, or watersports in no time. The 438559 fuel pump is the standard replacement for these motors, and it is widely available through aftermarket suppliers like KEMSO.
The key takeaway is this: do not settle for a cheap, untested pump that may fail again in a few months. Invest in a KEMSO fuel pump for quality, reliability, and peace of mind. With a lifetime warranty and USA-based support, you can trust that your engine will run strong for years to come. For the best selection and pricing, visit https://www.kemsoracing.com/ and find the right pump for your Johnson 70 HP motor today.