Horse power from a motorcycle drag racers point of view.
I measure HP in .001 seconds and .001 MPH changes.
The things that change these figures are temprature and altitude and humidity. Since it appears that you are sailing on the ocean and not a lake at 6,000 ft. here is my guess.
Carburator jetting, I think you are running lean.
The pump gas is not just gas any more. It has some amount of alcohol in it. It takes twice as much alcohol as gas to make the same amount power.
The more alcohol the leaner the motor will run so a bigger jet will be required. E-85 will require almost twice the amount fuel (by weight) than straight gas.
You have the equipment and the mindset to try rejetting. Just get the two next size larger jets for the carb and see what happens.
Fuel injected motors with oxegen sensors can modify their fuel curves via the ecu but carburator engines cannot.
I just think you motor is nosing over because it is to lean to make the hp.