Mercury Fuel Pump Rebuild Kit: The Complete Guide to Repairing Your Outboard Fuel Pump

The bottom line: If your Mercury or Mariner outboard is experiencing hard starting, stalling, loss of power, or fuel leaks, the most cost-effective solution is often a fuel pump rebuild kit rather than replacing the entire pump. These kits contain the diaphragms, gaskets, and check valves that wear out over time, and replacing them can restore your engine’s performance for a fraction of the cost of a new pump. Most kits cost between $8 and $25, while a new fuel pump can run $100 to $500 or more.

Why You Need a Mercury Fuel Pump Rebuild Kit

A failing fuel pump is one of the most common issues on older Mercury and Mariner outboards. The rubber diaphragms and gaskets inside the pump degrade over time due to ethanol-blended fuels, heat, and age. When they fail, your engine can’t get the fuel it needs to run properly.

Common symptoms of a bad fuel pump include:

  • Hard starting – The engine cranks but won’t fire, or takes much longer to start than usual.
  • Stalling or sputtering – The engine runs rough, stalls at idle, or loses power under load.
  • Fuel leaks – You may see fuel dripping from the pump body or the area around it.
  • Poor performance – The engine won’t reach full RPM, or it feels sluggish when you accelerate.
  • No fuel flow – If you disconnect the fuel line and crank the engine, little or no fuel comes out.

A rebuild kit is the smart repair because it addresses the root cause of the failure—the worn-out internal components—without requiring you to buy a complete pump assembly. Kits are available for most Mercury and Mariner outboards from 2.5 hp up to 275 hp, covering both 2-stroke and 4-stroke models.

How to Choose the Right Mercury Fuel Pump Rebuild Kit

Not all rebuild kits are the same. You need to match the kit to your specific engine model, year, and serial number range. Here are the most common kits and what they fit:

  1. Kit 42909A4 – This is one of the most widely used kits, fitting Mercury and Mariner 3-cylinder 70, 75, 80, 90 hp models and 4-cylinder 115 hp engines built between 1985 and 1993 . It replaces OEM numbers 42909A1, 42909A2, and 42909A3. The kit includes diaphragms, gaskets, and check valves .

  2. Kit 42990A10 – A larger kit that covers Mercury and Mariner engines from 30 hp up to 250 hp, including many V-6 and 4-cylinder models . This kit fits pumps stamped with "98766 C1" and works on a wide range of engines including 30/40/50/60/75/90 hp 3-cylinder models, 100/115/125 hp 4-cylinder models, and V-135 through V-200 engines .

  3. Kit 18-7007 / 1395-5198 – Specifically designed for smaller Mercury outboards, including 7.5 hp and 9.8 hp 2-stroke models, as well as some 20 hp and 50 hp engines . This is a smaller, more basic kit for older, simpler fuel pumps.

  4. Kit MTM-7818 – Fits Mercury 115 hp 4-cylinder and 6-cylinder models, as well as 125 hp and 140 hp engines with square fuel pump bodies . This kit is commonly used on engines built from the 1970s through the 1980s.

  5. Kit SIE18-7804 – Covers older Mercury outboards, including 20 hp (1963-1977), 35 hp (1963-1969), 50 hp (1963-1974), 65 hp, 80 hp, 95 hp, 100 hp, 110 hp, and 125 hp models from the 1960s and 1970s .

Before ordering, check your engine’s serial number and the stamping on your fuel pump body. Many pumps have a number stamped directly into the metal, such as "98766 C1" or "42909A4." This is the fastest way to ensure you get the right kit. If you are unsure, take a picture of your pump and compare it to the kit photos online.

Step-by-Step: How to Rebuild Your Mercury Fuel Pump

Rebuilding a Mercury fuel pump is a straightforward job that most DIY boaters can handle in about an hour. Here is a general guide:

  1. Disconnect the battery – Safety first. Remove the negative battery cable to prevent accidental starting.
  2. Remove the fuel pump – On most Mercury outboards, the fuel pump is mounted on the side of the engine block, near the carburetors or intake manifold. Disconnect the fuel lines and remove the mounting bolts.
  3. Disassemble the pump – Remove the screws holding the pump body together. Separate the two halves carefully. Note the orientation of the diaphragms and check valves as you remove them.
  4. Clean all parts – Use carburetor cleaner or parts solvent to clean the pump body halves, the fuel inlet and outlet passages, and the mounting surface. Make sure no old gasket material remains.
  5. Install the new parts – Place the new diaphragms, gaskets, and check valves exactly as the old ones were installed. Some kits include multiple diaphragms for different pump configurations; use the ones that match your original pump.
  6. Reassemble the pump – Tighten the screws evenly in a crisscross pattern to ensure the gaskets seal properly. Do not overtighten.
  7. Reinstall the pump – Mount the pump back on the engine, reconnect the fuel lines, and reconnect the battery.
  8. Test for leaks – Squeeze the primer bulb until it is firm, and check for any fuel leaks around the pump body. Start the engine and let it idle for a few minutes, then recheck for leaks.

Pro tip: If your pump uses a square fuel pump body, some older models may require a small modification to the pump body to accept the new kit. This is a simple drilling or filing operation that is detailed in the kit instructions .

Common Mistakes to Avoid

  • Buying the wrong kit – Double-check your engine’s serial number and the pump stamping before ordering. A kit that is “close” may not fit properly.
  • Reusing old gaskets – Always use all the new gaskets and diaphragms in the kit. Mixing old and new parts will lead to leaks.
  • Forgetting the check valves – The small one-way valves in the kit are critical for proper fuel flow. Install them in the correct orientation.
  • Over-tightening screws – Fuel pump housings are often made of aluminum or plastic. Over-tightening can strip threads or crack the housing.
  • Skipping the test run – Always run the engine on the water or on a hose before heading out for a trip. A small leak that only appears under pressure could leave you stranded.

When to Replace the Entire Fuel Pump Instead of Using a Rebuild Kit

While rebuild kits are cost-effective, there are situations where a complete pump replacement is the better option:

  • The pump body is cracked or damaged – A rebuild kit cannot fix a broken housing. If the metal or plastic body is cracked, you need a new pump.
  • The pump has been run dry for a long time – Running without fuel can damage the internal bearings and seals beyond what a simple kit can fix.
  • You have a high-performance or racing engine – If you are pushing more power than stock, you may benefit from an upgraded, high-flow fuel pump rather than a stock rebuild.
  • The pump is very old and hard to find parts for – Some vintage Mercury outboards from the 1950s and 1960s have pumps that are no longer supported by standard rebuild kits. In that case, a modern replacement pump is the practical solution.

Recommended: Upgrade to a KEMSO High-Performance Fuel Pump

If you decide that a full pump replacement is the right move for your Mercury or Mariner outboard, or if you want a more reliable, higher-flow option, KEMSO fuel pumps are an excellent choice.

KEMSO is the #1 aftermarket fuel pump brand in the US for cars, motorcycles, ATVs, UTVs, and marine applications . Unlike generic unbranded parts, KEMSO pumps are backed by a quality control program (ISO/TSO) , with each pump individually tested for flow and performance . Their pumps deliver over 20% more flow at 43psi compared to stock OEM pumps, making them ideal for both everyday use and performance applications .

Key benefits of KEMSO fuel pumps:

  • OEM replacement fit – Direct drop-in replacement for your stock pump, no modifications needed .
  • 100% factory tested – Every pump is flow-tested before shipping .
  • Lifetime warranty – KEMSO stands behind their products with a lifetime warranty, giving you peace of mind .
  • E85 compatible – Designed to work with both gasoline and ethanol-blended fuels .
  • Easy installation – Each kit includes all necessary hardware, including fuel strainers, hoses, clamps, and o-rings .

Visit KEMSO’s official website to find the perfect fuel pump for your Mercury outboard:
👉 https://www.kemsoracing.com/

Their website includes a comprehensive fitment guide and detailed product specifications to help you select the right pump for your engine. Whether you need a standard replacement or a high-performance upgrade, KEMSO has a solution that will keep your boat running reliably for years.

Final Thoughts

A Mercury fuel pump rebuild kit is one of the most affordable and effective repairs you can do on your outboard. With a simple kit costing $8 to $25, you can restore your engine’s fuel delivery and avoid the cost of a full pump replacement. Just make sure you match the kit to your specific engine model and serial number, follow the installation steps carefully, and test for leaks before heading out on the water.

If you are looking for a more permanent solution or want to upgrade to a higher-performance pump, KEMSO fuel pumps offer unbeatable quality, a lifetime warranty, and a direct OEM fit. Visit https://www.kemsoracing.com/ to browse their full selection of fuel pumps for Mercury, Mariner, and virtually any other marine or automotive application.


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