SouthMarine Fuel Pump Diaphragm Kit 42990A10 for Mercury 35–275HP — Direct-Fit OEM Seals

SouthMarine Outboard 42990A10 Fuel Pump Diaphragm Kit replaces worn diaphragms and seals in Mercury outboard fuel pumps for engines from 35HP up to 275HP.

This kit is a direct-fit replacement designed to restore proper fuel flow and pressure in a range of Mercury outboards. It contains precision-cut diaphragms and associated sealing components matched to OEM part numbers so you can repair the fuel pump without replacing the entire assembly. The kit is compatible with many numbered fitments and covers a broad spread of horsepower ratings, making it useful for owners, mechanics, and marinas servicing mid- to high-power outboard motors.

Which Mercury engines and part numbers does this diaphragm kit fit?

The kit is built to match multiple OEM references and engine outputs so you can confirm compatibility quickly. It covers a long list of fit part numbers and is suitable across many Mercury horsepower classes commonly found on recreational and commercial boats.

• Compatible fit numbers include 21-42990A1 through A11 and 21-857005A1 variants.
• Engine coverage spans 35HP, 40HP, 45HP, 50HP, 55HP, 60HP, 65HP, 75HP, 80HP, 90HP, 100HP, 115HP, 125HP, 135HP, 150HP, 175HP, 200HP and 275HP.
• Cross-references Sierra 18-7817 where applicable for alternative sourcing.
• If uncertain, supply your engine serial number to verify fitment before purchasing.

What components are included and how do they restore pump function?

The kit replaces the flexible diaphragm and the small seals that control fuel movement inside the pump. Renewing these parts eliminates common causes of leaks, poor priming, and inconsistent fuel delivery, helping the pump regain proper operation.

• Precision diaphragms manufactured to match original dimensions and flex characteristics.
• Seals and O-rings for inlet/outlet ports to prevent fuel or air ingress.
• Gaskets or backing plates where required for a secure assembly.

How easy is installation for a DIY mechanic?

Installation is straightforward for anyone with basic mechanical skills and a few hand tools. Follow a service manual or step-by-step guide, keep the work area clean to avoid contaminating the fuel system, and replace parts one at a time to ensure correct reassembly.

• Remove fuel pump cover and note orientation of parts before disassembly.
• Clean mating surfaces and inspect housing for wear or damage.
• Install new diaphragm and seals, torque fasteners to manufacturer specs.

When should you choose a diaphragm kit over a full pump replacement?

Choose the diaphragm kit when the pump housing and drive components are in good condition but fuel delivery issues stem from perished rubber parts. A kit is a cost-effective repair that restores performance without the expense of a complete pump swap.

• Use when leaks, soft primer bulbs, or inconsistent idle are traced to worn diaphragms.
• Replace diaphragms routinely if the engine sees many service hours or old fuel exposure.
• Opt for a full pump replacement if the housing, valves, or drive linkage show corrosion or mechanical failure.

Who this kit is for: Mechanics, boat owners, and marine service centers who maintain Mercury outboards across a wide horsepower range will find this SouthMarine diaphragm kit a practical repair option. It’s aimed at those who prefer economical, accurate replacements that match OEM fitments and restore reliable fuel pump performance without replacing the entire unit. If you’re unsure about fitment, provide your engine serial number to confirm this kit is the correct match for your outboard.