Re: Nissan / Tohatsu 6h 4 stroke
Rejetting the carbs isn't rocket science. It is routinely done on motorcycles. Every bike you see with aftermarket exhaust (that means probly 75% of em) has been rejetted- or should have been. There are kits for the purpose.
I work on bikes which have 4 carburetors. Tear em all apart and put em back together. And these boat engines don't have the constant velocity system. At least not the ones I've seen.
I'm sure Tohatsu doesn't have a carb specifically designed for this motor. They select from what is produced by Mikuni or Keihin or whoever. I bet its an even chance that the motors which run well have a different carb than the poor running ones.