Hey Crosbyman – Marineengine.com lists the Champion plug for a 1974 9.9 as UL-81C, not L82C, but I’ve seen disagreement before as to where you go to look spark plug usage up so I’m not too surprised you have a different number. . They list the L82C for 1977 and newer. – probably a wider gap plug for the higher energy spark.
For the problem motor I fixed with NGKs , I used BR7HS plugs to cure it, which is the UL-81C equivalent,
Marine engine’s usage chart below : https://www.marineengine.com/parts/champion/outboard/johnson-outboard-spark-plug-guide.html
Dave
