Hard to see all that way to Cape town but 22x8 MenzS is a BIG load on that engine
my off hand guess is overloaded --due to prop load - lean mix- restrictive muffler- low oil% -poor cooling flow
any or all of these
PS it is almost IMPOSSIBLE to over rev the engine - ideally you prop it to run at least 6500 on the ground easily -no matter what the prop used.
running easily of a mix which will easily four cycle , is important to do
Once you have established that full throttle can be made to run rich and lean by shifting high speed needle - you are on your way to being in correct rpm for the prop