BEST plug for 1981 Merc 7.5

[pvanv]

Im going through this OB, s/n 5848475, to get ready for spring. I believe this is a 1981 model year, based on the serial number, according to a few websites.

The plugs presently installed are Champion L77JC4's. The plugs look like they may have been running a little on the hot side. Not bad, but unusually clean ceramic on the center electrode. Is this the specified plug for this motor?

The NGK plug book calls for a B9HS-10 based on the application, but cross-references the Champ L77JC4 plug to two steps hotter, B7HS-10.

I wonder if I should stay with the L77JC4's, or go a step colder, more like the NGK application suggests.

The motor is used as a sailboat auxiliary on an O'Day 23. It's about the right power, and doesn't seem to lug with the present prop.

Thoughts?

 

[Laddies]

Re: BEST plug for 1981 Merc 7.5

Never use the cross cross reference to select a plug go to the manufactures recommendation. Your engine is a 1981 as you thought and should use a B9HS-10 or a Champion L77J4C/QL77J4C.

 

[emckelvy]

Re: BEST plug for 1981 Merc 7.5

You're pushing quite a good load with that motor, and like you said the plugs look like they've been running hot. Probably a good idea to make sure your cooling system is up to snuff.

You might also consider running a surface-gap plug, which will run a bit cooler than the standard electrode-type.

Older 7.5hp models used a Champion L78V surface-gap spark plug or equivalent: AC V40FFK; NGK BUHW-2; Autolite 2872 .

HTH............ed

 

[pvanv]

Re: BEST plug for 1981 Merc 7.5

Thanks guys. All good information.

This re-confirms what I was thinking. I usually work on different (non-Merc) brands, and was surprised how difficult it was to cull information on this model (from the Internet) without manuals.

Thanks again.