If your Mercury, Mariner, or Force two-stroke outboard is hard to start, sputters, stalls, loses power, or struggles to maintain fuel delivery under load, the manual fuel pump may need service.
Many older outboards use a crankcase pulse-style diaphragm fuel pump. Instead of an electric motor, this pump uses pressure pulses from the engine crankcase to move a flexible diaphragm, drawing fuel from the tank and delivering it to the carburetors or fuel system through internal check valves.
Over time, the diaphragm, gaskets, seals, and check valves can harden, crack, shrink, or become contaminated by old fuel, ethanol residue, debris, or age. When that happens, fuel flow becomes weak or inconsistent. A fuel pump rebuild kit is a cost-effective way to restore the original pump body instead of replacing the complete fuel pump assembly.
Select
1) Later Square Style 42990A10 Fuel Pump Repair Kit
The later square-style Mercury fuel pump repair kit is commonly searched under Mercury part numbers 42990A1, 42990A2, 42990A3, 42990A4, 42990A5, 42990A6, 42990A7, 42990A8, 42990A9, 42990A10, 42990A11, and 21-857005A1.
The 42990A10 and 857005A1 fuel pump diaphragm kit family is widely used on Mercury, Mariner, Force, Chrysler, and US Marine two-stroke applications that use the square manual diaphragm fuel pump design.
This style also cross-references with popular aftermarket numbers including Sierra 18-7817, S18-7817, and GLM 40290. Many customers refer to this as the Mercury 42990A10 fuel pump repair kit, Mercury 857005A1 diaphragm kit, or Sierra 18-7817 fuel pump rebuild kit.
2) Earlier Oblong Style 9036A1 Fuel Pump Repair Kit
Some earlier Mercury, Mariner, Force, Chrysler, and Sport Jet applications use the older oblong-style manual fuel pump instead of the later square pump. For those engines, select the earlier oblong-style 9036A1 fuel pump repair kit.
Because many older motors have been repaired or converted over the years, the most reliable way to choose the correct kit is to compare the shape of your existing fuel pump, verify the part number on the pump or parts diagram, and confirm fitment by engine serial number.
Broad Mercury, Mariner & Force Fitment
This fuel pump rebuild kit listing supports a wide range of Mercury, Mariner, and Force two-stroke outboards from approximately 30 HP through 300 HP, depending on pump style and engine configuration.
Applications include many 2-cylinder, 3-cylinder, 4-cylinder, inline, and V6 outboards, including 2.0 Liter, 2.4 Liter, 2.5 Liter, and 3.0 Liter Mercury V6 engines. Common horsepower ranges include 30 HP, 40 HP, 50 HP, 60 HP, 75 HP, 90 HP, 100 HP, 115 HP, 125 HP, 135 HP, 150 HP, 175 HP, 200 HP, 225 HP, 250 HP, 275 HP, and select 300 HP applications.
This makes the kit useful for everything from smaller Force and Mariner fishing motors to high-performance Mercury V6 outboards. Because Mercury used several pump styles across different years and serial-number ranges, always select the correct version by pump body shape, part number, and engine serial number.
What the Fuel Pump Rebuild Kit Does
A manual outboard fuel pump depends on a flexible diaphragm and internal check valves to move fuel. When the diaphragm is soft and sealing correctly, crankcase pressure pulses flex the diaphragm and allow fuel to move in one direction toward the engine.
When the diaphragm becomes stiff, torn, or fuel-soaked, the pump may no longer maintain steady fuel delivery. Weak check valves can allow fuel to bleed back, causing hard starting, long cranking, poor idle, hesitation, or loss of power at higher RPM.
Rebuilding the pump replaces the wear components inside the existing pump body, helping restore fuel flow, improve starting, stabilize idle quality, and reduce fuel starvation problems.
Common Symptoms of a Weak Fuel Pump
A failing diaphragm-style fuel pump can cause hard starting, poor idle, bogging on acceleration, random stalling, surging at cruise, fuel starvation at wide-open throttle, or a motor that runs better when the primer bulb is squeezed.
If the primer bulb temporarily improves the way the engine runs, the fuel pump, fuel lines, filters, tank pickup, anti-siphon valve, or air leaks in the fuel system should be inspected.
A rebuild kit is often the right repair when the pump body is still in good shape but the internal diaphragm, gaskets, seals, or check valves are worn.
Installation and Service Notes
Before rebuilding the pump, clean the work area and note the order of every diaphragm, gasket, valve, spring, and plate during disassembly. Old fuel residue and debris should be removed from the pump body, but sealing surfaces must not be gouged or damaged.
Inspect the fuel lines, primer bulb, fuel filter, tank pickup, fittings, and clamps at the same time, because a rebuilt pump cannot overcome air leaks, restricted fuel lines, or a plugged tank vent.
After installation, prime the fuel system, check carefully for leaks, and verify that the engine starts, idles, and accelerates normally. On performance Mercury V6 engines, confirm fuel delivery under load before extended high-RPM operation.
Selecting the Correct Kit
This listing includes one fuel pump rebuild kit. Choose the later square-style 42990A10 / 857005A1 fuel pump repair kit if your engine uses the square diaphragm-style pump. Choose the earlier oblong-style 9036A1 fuel pump repair kit if your engine uses the older oblong pump design.
Matching the kit to the pump body is critical because diaphragms and gaskets are not interchangeable between different pump styles.
Whether you are maintaining a 30 HP Force, rebuilding a 90 HP Mariner, servicing a Mercury 150, repairing a 200 HP V6, or keeping a high-performance 2.5 Liter or 3.0 Liter Mercury running strong, a properly rebuilt manual fuel pump is essential for dependable fuel delivery.
The Buckshot Racing #77 Mercury, Mariner, and Force fuel pump rebuild kit gives boaters, racers, and marine technicians a practical way to restore fuel pump performance without replacing the entire assembly.