Stepping back slightly form the problem it sounds like it;s not making max available RPM - Zapcats were designed to runs with Tohatsu 50's? I would imagine even if you messed up the trim spectacularly, it should make a reasonable effort to ge to max RPM. As someone said earlier - a 3- cyl 2 -stroke will not sound spectacualrly rough with only 2 cyls firing, but due to the two remaining cyls having to compress the air, only to get nothing back out in the form of a "bang", it feels more like half than 2/3 throttle.
As an example when i got the Merc it ran Ok at idle, but when I first opened the throttle it did a half hearted impression of a 25. Took the plugs out, bottom one was literally dripping with oil, middle was not so bad but still manky. Changed them, puttered out the marina (even sounded better at tickover) pushed the lever forward & was nearly thrown off the back of the seat!
Moral of the story: Spark plugs are cheap, and don't like being drenched in oil! I'd buy a new set of plugs. What's the worst that can happen? - you end up with a spare set for the next service.
If you are towing, a good idea is to pull the fuel connector before you come ashore or when you are flushing & let the engine drink the contents of the carb bowls. It's not a problem with auto mixers, but also saves oil build up as the fuel evaporates if running premix.