Sounds like a tune or engine issue. It should handle the pump fine.
First, are you sure it is '9 hp'? Honda, and several others, and probably the china copies, have taken to putting big letters on the shroud 5 or 6 or 5.5, that has nothing to do with hp. I have a small Honda '5.5' that on the website is actually 4.6 hp at 3600, and doesn't reach 5 hp even at 4000 rpm. Dishonest, but technically legal marketing. Gee, we didn't say hp, you just assumed that.....
Second, do you know the pressure settings of the main RV, and of the unloading valve in the pump? Those may be too high, requiring too much hp to turn the pump.
Tons is cylinder size (area) times operating pressure. It has nothing to do with pump size or engine hp. It can be a hand operated piston pump or a car engine, but pressure times area is force. So, cylinder size directly defines the tons of the splitter. Many posts here about mfr uprating the tons, when the cylinder size would require much higher pressure to actually get what they advertise. A polite way of saying marketing lies and BS.....
Next, pump size and speed (gpm) define speed. Flow divided by the cylinder area defines how fast the cylinder moves.
Bigger pump = more flow = faster, but same force or tons.
Part throttle or full throttle = more engine rpm = faster, but same force or tons. (unless there is serious leakage)
Then, operating pressure times flow defines the hp needed to turn that pump at that pressure. If the minimum hp is met to turn the pump, adding more hp to the engine won't change force or speed.
It could however allow a higher unloading pressure setting before the engine runs out of power on the high flow low pressure setting, thus carry the same high speed on heavier loads before it slows down. But by itself, more engine hp won't give you more speed or more force unless something else is changed.
