I've always used j19lm equivalent ngk br2-lm. Neighbour brought his to check bad running (i think auto choke not opening). He put new plug in, ngk br6s, resistor version of j8c. Is that an okay plug for the quantum 6.75 hp? Can't see code date without removing covers.
Update: found briggs site .quantum before 11nnnnn took j19lm or cj8. No mention of j8c. geez . Just hex wrench size difference.