Spark Plugs for Thunderbolt Ignitions

[RogerH]

My findings: The 69-76 Merc 650/800/850 Thunderbolt CD-Distributor ignitions are pretty cost effective and provide excellent performance for replacing Mags in earlier 30/55H's and similar Mod motors. A good Mag is just fine in most cases, but their availability and parts (coil = $110) is the real problem. Excellent CDI and Seirra switchboxes and coils are readily available.

The question: What type of spark plugs perform well in the early motors when converted to Thunderbolt ignition?

I would suspect the heat range should be about the same, but I'm wondering whether polar gap plugs would perform well in these earlier engines. Possibly there is another electrode style that should be tested. Any suggestions / help would be appreciated.

[nbranews]

spark Plugs

We run the Mercury Thunderbolt on our 44 DM and we use the Champion L77JC4(common OMC plug).According to Mercury the reccomended plug for these engines is the Champion L76V or NGK BUHW. Depending on what mods have been done to your engine you will have to experimant and see which plug burns the best. Champion Plugs the higher the number the Hotter. NGK plugs the Lower the number the hotter.

[sam]

I have not found any plugs to be superior to others with Thunderbolt Ignition. If it fits in the hole and has an isolated electrode, it seems to work. The only time I use new plugs is to take a carb jet reading, they seem to last forever with the short amount of time put on them racing.