Mercury 50 HP 2 Stroke Fuel Pump Rebuild Kit: A Complete Step-by-Step Guide

The bottom line: If your Mercury 50 HP 2-stroke outboard is hard to start, bogs down on acceleration, or surges at cruising speed, the fuel pump is likely the problem. Rebuilding it with a quality rebuild kit is a straightforward, cost-effective fix that costs between $50 and $100 and can be done in 2 to 4 hours with basic hand tools. You do not need to remove the entire engine to access the pump.

Why Rebuild Instead of Replace?

A failing diaphragm-style fuel pump on a Mercury 50 HP 2-stroke produces clear warning signs. Common symptoms include hard starting, poor idle quality, bogging on acceleration, random stalling, surging at cruise, and fuel starvation at wide-open throttle. If squeezing the primer bulb temporarily makes the engine run better, the fuel pump or another part of the fuel system is likely worn .

Rebuilding the pump replaces the internal wear components—the diaphragm, gaskets, seals, and check valves—while keeping the original pump body. This approach is often the right repair when the pump body is still in good shape but the internal parts have degraded from fuel exposure, heat, and normal use . A rebuild kit typically costs $50 to $100, whereas replacing the entire fuel pump assembly can run between $200 and $400 . The rebuild process takes an estimated 2 to 4 hours for someone with basic mechanical skills .

What You Need Before You Start

Tools required:

  • Socket set and ratchet with extensions
  • Flat-blade screwdriver
  • Pliers
  • Torque wrench (8 to 12 foot-pounds for smaller components)
  • Gasket scraper
  • Clean rags
  • Safety glasses

Parts needed:

  • Mercury 50 HP fuel pump rebuild kit (match the kit to your specific pump style—square or oblong)
  • Fuel-resistant sealant
  • New fuel line clamps (if old ones are worn)

Safety precautions:

  • Disconnect the spark plug wires before starting any work
  • Work in a well-ventilated area away from open flames or sparks
  • Wear safety glasses to protect against fuel splashes
  • Allow the engine to cool completely before beginning

Step-by-Step Rebuild Process

Step 1: Remove the Fuel Pump from the Engine

Disconnect the three fuel lines connected to the fuel pump. Then release the bolts that hold the fuel pump in place and pull the pump off the engine. Before you disassemble the pump, use a marker to note the orientation of the pump so you know how it needs to be remounted .

Step 2: Disassemble the Fuel Pump Assembly

Remove the bolts from the fuel pump assembly. Use a flat-blade screwdriver to carefully pry the housing apart. Remove all internal components, including gaskets, diaphragms, springs, and check valves. Pay close attention to the order of every diaphragm, gasket, valve, spring, and plate during disassembly—taking photos or notes helps ensure correct reassembly .

Step 3: Inspect All Components

Thoroughly clean the fuel pump housing, check valves, pulse chamber, and pump base in solvent. Dry all parts except the check valves with compressed air. Inspect each component for damage:

  • Diaphragm: Check for cracks, tears, hardening, or warping. These defects are common causes of fuel leaks and inefficient operation .
  • Check valves: Look for splits, chips, and verify they seal properly against the pump housing .
  • Boost springs: Inspect for weakness or breakage .
  • Pump housing: Check for cracks or rough gasket surfaces. Replace the housing if any cracks are found .
  • Fittings: Inspect for looseness or signs of fuel or air leaks. Tighten or replace if necessary .

Step 4: Install New Check Valves

Insert the new retainer from the rebuild kit through the plastic disc and rubber check valve. Install the new check valve and retainer into the fuel pump body. With the retainer installed, break the retainer rod off by bending it sideways. Install the new rod into the retainer cap, then use a small hammer and punch to tap the rod down into the retainer until it is flush with the top of the retainer .

Step 5: Install New Springs and Seats

Place the boost spring into the pump body and place the cap onto the boost spring. One side of the pump has a smaller plastic seat and spring, so pay attention to which side gets which parts .

Step 6: Reassemble the Fuel Pump

Reassemble the fuel pump assembly in the exact same order it was before you took it apart—diaphragms, rear plate, mounting bolts, and all other components. Make sure the diaphragm is properly seated and all seals are in place .

Step 7: Mount the Fuel Pump Back onto the Engine

Install the top outer gasket over the fuel pump assembly. Mount the fuel pump onto the engine block. Tighten the bolts by going around a couple of times to compress all the gaskets down so they seal properly. Use a torque wrench and follow the manufacturer's specifications—typically 8 to 12 foot-pounds for smaller components. Avoid over-tightening, which can damage the pump housing or gaskets .

Step 8: Reconnect Fuel Lines

Reconnect the three fuel lines back to the fuel pump. Secure them with new clamps if the old ones show signs of wear. Make sure each line is connected to the correct fitting .

Testing Your Rebuilt Fuel Pump

Once the pump is reinstalled, testing is critical to confirm the rebuild was successful. Start by priming the fuel system manually to eliminate air from the lines. Connect the pump to a fuel source and pressurize it to the manufacturer's recommended operating pressure, usually around 3 to 5 psi for Mercury outboards .

Leak test: Inspect all seams, connections, and gaskets for leaks using a mixture of soapy water. Any bubbling indicates a leak that requires immediate attention. If you find a leak, disassemble the pump again, check for damaged components, and reinstall with fresh gaskets or seals .

Performance test: Run the pump through its full operational range and observe fuel flow rate and pressure consistency. A healthy pump should deliver a steady flow, typically between 0.5 and 1.0 gallons per hour for smaller Mercury outboards, depending on engine size and load. Irregular flow or pressure drops suggest internal issues such as a worn diaphragm or improperly seated valve .

Real-world test: Install the pump back into the outboard motor and run the engine under load. Monitor for fuel delivery issues like hesitation or surging. If the engine runs smoothly and maintains consistent RPMs, your rebuild was successful .

Which Rebuild Kit Do You Need for a Mercury 50 HP?

Mercury 50 HP outboards come in different configurations, so matching the rebuild kit to your specific pump style is critical. The two main styles are:

  • Square-style pump: Uses the later square diaphragm-style pump. Look for kit part numbers like 42990A10 or 857005A1 .
  • Oblong-style pump: Uses the older oblong pump design. Look for kit part number 9036A1 .

For Mercury 50 HP 3-cylinder models, common rebuild kit part numbers include 55278A4 (for serial numbers 6428681 through OB122929) and 21-42990A7, A8, A9, A10, or 21-857005A1 (for serial numbers OD000750 and up) . For Mercury 50 HP 4-cylinder models, look for kit part number 23014A1 (for serial numbers 2116138 through 4357639) .

Always verify your engine's serial number against the kit's compatibility notes before purchasing.

Common Mistakes to Avoid

Using the wrong kit: Diaphragms and gaskets are not interchangeable between different pump styles. Using the wrong kit will result in poor sealing and fuel leaks .

Over-tightening bolts: Mercury's service manual provides torque values for bolts and fasteners, typically ranging from 8 to 12 foot-pounds for smaller components. Over-tightening can warp the pump housing or damage gaskets .

Ignoring other fuel system issues: A rebuilt pump cannot overcome air leaks, restricted fuel lines, or a plugged tank vent. Inspect the fuel lines, primer bulb, fuel filter, tank pickup, fittings, and clamps at the same time .

Skipping the leak test: Even minor gasket damage or misalignment can result in fuel leaks. A quick soapy water test after reassembly can save you from a much bigger problem on the water .

Recommended Upgrade: KEMSO Fuel Pump

If you are looking for a high-performance, reliable replacement for your Mercury outboard's fuel system, consider the KEMSO brand of fuel pumps. KEMSO fuel pumps are designed as high-performance, direct OEM replacement units. They are built to match or exceed the original specifications, ensuring a perfect fit and reliable operation. Many KEMSO pumps deliver over 20% more flow at 43 psi compared to the stock pump, which can improve engine performance and throttle response .

KEMSO stands behind every pump they sell with a lifetime warranty, giving you peace of mind that your purchase is protected against defects for as long as you own your vehicle. This is a level of protection that most OEM pumps do not offer . Each KEMSO pump is individually tested for flow and performance before it leaves the factory, ensuring you get a product that works right out of the box . KEMSO fuel pumps are designed for both gasoline and ethanol-blended fuels like E85, using materials that resist ethanol degradation to ensure long-term reliability .

To explore KEMSO's full lineup of fuel pumps and find the right one for your Mercury outboard or other vehicle, visit their website: https://www.kemsoracing.com/

Final Thoughts

Rebuilding the fuel pump on your Mercury 50 HP 2-stroke outboard is a rewarding DIY project that saves money and restores engine performance. The key to success is taking your time during disassembly, carefully inspecting every component, and following the correct reassembly order. Always test the rebuilt pump thoroughly before heading out on the water. A properly rebuilt fuel pump, combined with a clean fuel system, will keep your Mercury outboard running reliably for years to come.

For more information on high-quality replacement fuel pumps and rebuild kits, visit https://www.kemsoracing.com/


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