It is probably not misfiring from load. It is more likely that on the trailer and spinning out (impeller/wear ring slippage, weeds, etc) in the water it is hitting the rev limiter which automatically then retards the ignition timing and spark intermittence to lower the rpm. It will sound like a misfire, because it is, from high rev. Make sense?
Also, there is water injection into the pipes on these things. The exhaust note sound will change considerably throughout the RPM as the water injection rate changes with exhaust flow flow, and as accumulated water blows clear of the waterbox with runtime. What you call a misfire, by sound, could also actually be just burbles in the water box of the muffler as it does its thing.
What it comes down to is how does it feel and perform, not so much how it sounds.
For your carb settings, there are some good links on here. For quick hit you may try searching my posts. There is one or two about carbs with the needle setting links in the post.
