Mercury Primer Bulb Assembly 858763 – OEM Fuel System

Mercury Primer Bulb Assembly 858763

Part Number: 858763
Brand: Mercury Marine (Quicksilver OEM)
Official Product Type: Primer Bulb Assembly
System: Fuel Delivery System
Condition: New OEM

What It Does

The Mercury 858763 is a genuine OEM primer bulb assembly used in the fuel delivery system on selected Mercury and MerCruiser applications. The primer bulb is used to manually prime the fuel system before starting — drawing fuel from the tank to the engine and purging air from the fuel line. At $142+ as an OEM assembly, this is a complete primer bulb unit (not a simple squeeze bulb), typically including the bulb body, check valves, and fittings as an integrated assembly.

Compatible Applications

• Selected Mercury outboard engines — verify by model and serial number
• Selected MerCruiser sterndrive applications — verify by model and serial number

Serial Number Note: Primer bulb assemblies can vary by engine model, fuel system configuration, and production serial range. Always confirm this part number against the Mercury Parts Catalog using your exact engine model and serial number before ordering.

Supersedes / Alternate Part Numbers

• 858763 — current Mercury OEM part number (no known supersession on record)

If your parts diagram references a different number, contact us to confirm the correct part for your application.

Kit Contents

Complete OEM primer bulb assembly as supplied by Mercury Marine. Refer to your parts diagram for the exact components included in this assembly number.

Service Interval

Replace when the primer bulb is cracked, hardened, not holding pressure, or fails to draw fuel. Fuel system components should be inspected annually as part of routine engine maintenance. A primer bulb that won’t prime or collapses under use is a sign of internal check valve failure or bulb deterioration.

Technical FAQ

Q: How do I know if my primer bulb assembly needs replacement?
If the bulb feels hard or brittle, won’t firm up when squeezed, collapses after priming, or fuel leaks from the assembly, it’s time to replace it. A failed primer bulb can cause hard starting or fuel starvation under load.

Q: Is this a complete assembly or just the bulb?
At this price point, 858763 is an OEM assembly — not a simple aftermarket squeeze bulb. It includes the integrated check valves and fittings. Confirm the exact contents against your parts diagram.

Q: Can I use an aftermarket primer bulb instead?
Aftermarket options are available at lower cost for some applications. See the related products below for alternatives. For warranty-sensitive or newer engines, OEM is the recommended choice.

Related Products

• Mercury Primer Bulb 8M0061876
• Seachoice 21391 Low Permeation Fuel Line Kit with High-Output Primer Bulb – Mercury 3/8" x 7 ft.
• Attwood Fuel Line Hose Kit 3/8" x 6’ with Fuel Demand Valve – Mercury
• Fuel Kit Mercury 6’ x 3/8"
• Universal Outboard Fuel Line Assembly – 80" (3/8")

Parts Desk Insights

Best Use Case: This OEM assembly is the right choice when you need an exact factory replacement — particularly on newer Mercury engines still under warranty, or when the original assembly has proprietary fittings that don’t match standard aftermarket bulbs. It’s also the safest option when you can’t identify the exact fuel line diameter or connector type and want to match the original precisely.

Common Mistakes: The most common mistake is ordering a generic primer bulb by hose diameter alone without checking whether the original uses a proprietary Mercury connector. If the fittings don’t match, the bulb won’t seat properly and can leak or pull free under suction. Also, don’t wait until the bulb completely fails — a primer bulb that’s slow to firm up or stays soft after priming is already compromised and should be replaced before it causes a no-start on the water.