Reply To: 1975 Mercury 7 1/2 HP Spark Plugs

[CHUCK]

My feeling is that the surface gap plugs require more volts/amps to fire reliably.  Hence they are typically paired with the electronic ignition systems like that 1975 Mercury would have.  So;  they should work if the ignition system is healthy.

Last year I was working on a 20 HP Mercury with that same ignition.  Surface gap plugs would barely run the engine but the J6J’s would do better.  Turned out my coils were bad.  I replaced them and then the surface gap plugs worked fine.

Would it have continued to run fine with the old coils and the 6J6’s?  Who knows……