I do not know what the comparison between the merc and the johnson is but the year.There is no standard that says all 72 year 2 cycles use the same fuel ratio.That is entirely up to the manufacter and the oiling requirements of there engine.Do not take specs from an Evinrude and apply them to a merc.

I may be way off base,just trying to haed off any problems for ya later later.

Something to think about on that merc.Those mercs were design to opperate in the great lakes not our salt water.A big problem they have is the steel drive shaft rusting and siezing up at the powerhead and the lower unit.Makes it impossible to replace the water pump!Had to through one away that simply needed a water pump because of this.It is imperitive that the motor gets flushed ten times more than you think it should.I flushed mine religously and it still froze.