Spark plug recommendations for 15HP Merc and 15HP evinrude

[davevaill]

Can someone offer up some advice on replacement spark plugs for both motors? part number and style. The merc will be used in J classes and the Evinrude in ASH and Kpro, AXH. thank you!!!

[GrandpaRacer]

I like NGK heat range 8, so something like B8HS. I forget if both engines have short threads, I think so. If you need the long thread style it will have an E for extended following the 8 heat range. They are available in many tip styles, I doubt that makes any difference but others will claim it is huge. 😀

[GrandpaRacer]

For example the long thread one would be B8ES.

[davevaill]

Thank you!!

[mercsami]

The Mercury 15 uses NGK BPZ8HS-10,gapped at .060.Not sure what the OMC uses.

[ricochet112]

The Mercury 15 uses NGK BPZ8HS-10,gapped at .060.Not sure what the OMC uses.

Those will work, but if you're gaping at .060, the recommended plug, BPZ8HS-15 will be close to .060 out of the box.
I tried Champion L92LCC, in the Merc, which has a massively long tip, but didn't notice any improvement.

In the OMC's we use L77JC4 or QL77JC4 Champion, or NGK B7HS-10 or BZ7HS-10

[pat]

You may be able to see some small RPM improvement on your Digitron testing Champion versus NGK and changes in "gap". The best (most expensive) way to see any change is on a Land & Sea water brake Dyno. Also look at "indexing" the plugs whereby the opening of the gap faces the intake port. You do this by playing with the compression washers i.e. varying the thread depth so the gap aligns to the intake side when you torque to 20 ft. lbs.

[ZUL8TR]

Yep did that indexing for gap to face intake side when I ran 25ssh with the 25ss Merc in the 70's using Champ L61Y projecting tip with cut back ground electrode plugs. CD ignition could throw the large gap.

[Team B&H]

ZUL8TR,

We ran L4J plugs in our 25SS Mercs but cut the entire ground electrode off so there was no need for indexing. The Merc ignition handled the huge gaps easily - something we inadvertently discovered when trying to dry out a flipped motor before kill switches were required.

[ZUL8TR]

Very interesting. I suppose you had excellent results. I did use big gap in the L61Y. Should have tried what you did.
I looked up in my Champion Performance Manual the L61Y has a deeper projecting center electrode than the L4J design.

[Fast Freddie]

OOPS!: "...inadvertently discovered..."

[davevaill]

Thank you, everyone, for the tips and info!

[Graham]

We used NGK B9ES with platinum tip. Always burned great and won Nationals with this plug.

[davevaill]

Thanks, Graham, was that for the Merc or OMC?