I would go with the NGK spark plug, I’ve worked on a couple of 9.9 Johnsons (same as the 15 except for carb) that just wouldn’t idle right with Champions and ran great with NGKs. Check again the number. The first site below equates a QL82C to an NGK BR7HS. Note that NGK plugs get colder as that number in the middle gets larger. I’ve noticed in the past there is some disagreement with heat range correlation between Champion and NGK depending on who’s chart you look at.
Davehttps://maxrules.com/OMCplugchart.html
Here is NGK’s part finder:
In that NGK part finder chart, scroll to the 15 HP motors and even though your model is not there, note that other motors that use the QL-82C, list the BR7HS as the NGK replacement.
I have two 1991 15s and one similar 1989 that came from a failed resort. They supposedly are 100 to 1 motors, but I run them at 50 to 1. I’ll be puttin one an a hydroplane soon and I’ll go with 25 to 1 because I double up the oil on anything I run on the hydroplane because they are running at rpms above the design point. I often have to go a heat range colder on the plugs for hydroplane use.
Dave
