When I go from half to full throttle it pins me in my seat. At half throttle I'm at 4000rpm at full throttle I'm at 5600 rpm. Sounds like it's over propped or extremely rich

Were these engines dynoed and what are the details of the build

This really boils down to the perception or reality of what is actually half throttle. I take the readings off the TPS sensor when making these calls. Someone else might use relative throttle position at the helm which may not be indicative of whats actually taking place at the TB or carb. Regardless the amount of power required still increases with the cube of the velocity.

Was using the 1st 2 barrels of the carb as 1/2 throttle.