With the infamous early Merc triples (that I run and love) the top cylinders tend to fry pistons on a regular basis: Of the dozen plus to come through my shop, only two had a good top piston. Merc's solution was to put .0785 jets in the top carb (there are only two carbs) and .072s in the lower. (Normal jetting was .074 in both.)

From running hell out of them, I find that the test plugs run about the same with this unbalanced jetting.

Jeff