Answer
May 10, 2025 - 02:51 AM
On a 1975 20 HP Mercury 2-stroke outboard, the reed valves are located directly behind the intake manifold, inside the crankcase housing, just before the air-fuel mixture enters the crankcase.
To get to them, you’ll need to:
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Remove the carburetor and intake manifold.
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Behind the manifold, you’ll see the reed block assembly bolted to the crankcase.
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The reed valves are small metal flaps inside that block—they control airflow into the crankcase and are crucial for proper fuel delivery in a 2-stroke.
If you’re chasing a bogging or hard-start issue and suspect reeds, look for broken or chipped petals, gaps when closed, or excessive carbon buildup. You can usually clean or replace them with the block off the motor—just be gentle if they're original.
In any case you can purchase this 1975 Mercury Mariner 20HP Outboard Service & Repair Manual for the complete information on the reed valves, as well as other repair procedures with step-by-step instructions, clear images, and exploded-view illustrations.
