It sounds OK to me. You're running on muffs and in neutral. Have you tried it in gear and in the water?
I agree with you on the idle but it also have something to do with the higher rpm`s. I will explain. on low idle there is not so much back pressure as there is at higher rpm`s there for the engine will idle fine (if they are not broken or cracked) and you will have no blow outs in the carb. But as soon you are hitting the gas and get to a higher rpm the pressure are much higher. when the flaps go back they have to be tight flat on the cage, if they don`t, you will have blow outs through the carbs on the return and you will also have fuel leftovers in the chamber. to much fuel means your rpm will be weak and will have less power and so on. I did see light ( openings) in the reed cage between cage and flaps. That could be the problem, this is the only thing I have not tried yet. after lot`s of reading after a tip from a guy, I took them out to see if they was ok. and after some more reading I also found out that they have to be total closed to work 100%.
How you are running it is not normal. The motor is designed to run with the prop under water because the water causes exhaust back pressure which it needs. When you rev it on the muffs you get a lean condition which is bad and could damage the motor and the motor never gets to operating temps. You may also have a rev limiter on that motor. There are a few reasons to use muffs diagnosing high rpm problems is not one of themOkay, I don`t have that and I don`t have any boat on the water yet, so there is nothing to do then? no way to get it to go normal again?