Taking Care of Your Mercury 2-Stroke Pt. 2: Spark Plugs

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This will be a multiple part series about how to take care of your aging early model 2-stroke outboard. Check back to further articles on other yearly maintenance items you can easily do yourself.

For this part of the Mercury and other two stroke outboard maintenance series I’ll be showing you internet dudes how to take care of another simple part that can keep your outboard running smoothly. Your spark plugs. Just like in your vehicle, the spark plugs in an outboard motor detonate the fuel mixture which causes combustion within the outboards timing cycle. Having these plugs working properly is paramount to having a clean burning and smooth running engine.

What You’ll Need:

  • NGK BPZ8H-N-10 (or BP8H-N-10) Spark Plugs (or applicable to your outboard, see the sticker on the block or your users manual)
  • Spark plug socket for socket wrench
  • Compression tester (if you want to do this while the plugs are removed)

The DIY:

Step 1: Remove the kill switch clip/lanyard from the kill switch rendering the engine unable to turn over.

Step 2: Find the spark plugs at the rear of the cylinder block.

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Step 3: Remove the spark plug boots from the spark plugs and remove spark plugs from the block with your spark plug socket and wrench.

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Step 3.5: Compression test cylinders (if applicable)

If you would like to do a yearly compression test on your outboard cylinders this is the opportune time to do it. Remove your spark plugs one by one instead of all at once and do a compression test on each cylinder while that cylinder’s plug is out. This can be done by screwing in the compression tester on the open cylinder spark plug’s threaded hole, releasing any current pressure in the tester, and then cranking over the engine with the kill switch clip removed for five seconds. Now look at your compression gauge and it should tell you the current pressure of the cylinder. As long as each cylinder is within 5% of each other the compression is good on the outboard.

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Step 4: Gap the new spark plugs to .040 with a spark plug gapper (or whatever size is applicable for your outboard).

Use the black clips on the outside to bend your spark plug’s nose to the desired gap by using light and even pressure on it with the clips.

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Step 5: Screw in and tighten your new spark plugs

Hand tight and then a small turn with your spark plug socket to snug them in and replace the spark plug boots.

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If the spark plugs were gapped correctly and you tightened them snug you should be good to go! Enjoy your new spark plugs and smoother running outboard.

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