MerCruiser Carburetor Base / Adapter Gasket 866027 | 27-866027

Carburetor Base / Adapter Gasket — Part 866027

This is the genuine OEM gasket that seals the joint between the carburetor and its intake adapter on selected MerCruiser carbureted engines. It prevents vacuum leakage at the carburetor base — a common source of rough idle and lean-running conditions on carbureted MerCruiser engines. On 3.0L TKS assemblies, one gasket is used; on selected 4.3L Alpha and Bravo TKS installations, two are required around the carburetor adapter assembly.

Part Number Reference

• Current OEM number: 866027 / 27-866027
• No separate superseding Mercury part number is documented in current OEM references

Compatible Applications

MerCruiser used several carburetor and adapter arrangements within the same engine family. Confirm fitment by engine serial number and the applicable carburetor / TKS diagram. Note quantity required before ordering.

• 3.0L / 181 CID Inline-4 — 3.0L GM 181, 3.0L Alpha One, 3.0L TKS, and other 3.0L carbureted applications using the Mercury TKS / Mercarb assembly (1 gasket per assembly)
• 4.3L / 262 CID V6 — Selected 4.3L Carb Alpha, 4.3L Carb Bravo, and TKS-equipped carbureted 4.3L applications (typically 2 gaskets per assembly — verify by diagram)

When This Part Needs Replacement

• The carburetor is removed from the adapter for any reason
• The adapter is removed from the intake manifold
• A vacuum leak is suspected at the carburetor base
• The gasket is crushed, torn, hardened, or fuel-soaked
• TKS or carburetor service disturbs the original gasket
• Idle quality remains poor after fuel and ignition checks and a base vacuum leak is confirmed

This gasket should be replaced whenever the carburetor joint is separated — reusing a disturbed gasket risks a vacuum leak and difficult diagnosis.

Related Parts

• Mercury MerCruiser Quicksilver Gasket Set Carburetor 810929004 — full carburetor gasket set
• Mercury MerCruiser Quicksilver Carburetor Check Ball Kit 809520 — Mercarb & TKS, 3.0L / 4.3L / 5.0L / 5.7L
• 807726A2 — Carburetor adapter kit on selected 4.3L applications (search by part number)
• 8M0084193 — Carburetor assembly, superseding 866140A02 on selected engines (search by part number)
• 866146002 — Air-horn gasket (search by part number)
• 8M0113181 — Carburetor stud (search by part number)
• 8267086 — Carburetor mounting nut (search by part number)
• 54090 — 90-degree elbow (search by part number)

Installation Notes

• Remove all old gasket material from both mating surfaces before installing
• Inspect the carburetor adapter for warping or damage
• Verify whether your specific engine uses one or two 866027 gaskets before ordering
• Keep the intake manifold opening covered while working to prevent debris ingestion
• Follow the applicable MerCruiser tightening sequence and torque specifications
• Check for vacuum leakage at the carburetor base after reassembly and before running the engine

Parts Desk Insights

Most common customer issue: Ordering one gasket when two are needed. On selected 4.3L TKS and Alpha/Bravo carburetor assemblies, Mercury specifies two 866027 gaskets around the carburetor adapter. Check your carburetor / TKS diagram for the quantity before placing your order. Getting this wrong means a second disassembly to add the missing gasket.

Best use case: Ideal for customers overhauling a carbureted MerCruiser 3.0L or 4.3L, chasing a rough idle or lean condition caused by a carburetor base vacuum leak, or doing a complete carburetor service. Pair with the full carburetor gasket set 810929004 if the carburetor itself is being disassembled at the same time.