Re: 1984 mercury 115 no top end
Well, if the weather can't account for the loss then you have to start with basic troubleshooting.
You need to check your compression - one bad cylinder could account for the loss.
If ok, then check for decent spark at all cylinders. In general if they all have good fire at 4000 rpms then the ignition is ok. At that point both high/low speed stator coils will be putting out max power. That alone leads me to believe it's not an ignition problem.
So if the motor is mechanically sound and the ignition is working, then you only have the fuel system left.
Any number of things can cause a loss of max rpms, but pretty high on the list would be either an air leak in one of the hose or a restriction (anything from a partially clogged tank vent, to a restricted tank pick-up to a clogged fuel filter or clogged carb jet). Another component high on the hit list would be the fuel pump.
A slight perforation in the fuel pump diaphram would also cause a high end loss.