http://www.rideonlawnmowers.net/Honda-lawn-mowers.html This video shows you how you can increase the engine rpm on a Honda lawn mower.
In this case my engine was only spinning at about 2800rpm, when in fact most small engines run at about 3600rpm. I check the manual that came with the lawnmower and it does state that this engine makes 4Kw or 5.5hp at 3600rpm.
After adjusting the engine speed up it cuts grass a thousand times better and it drives heaps faster too. You can finf out more information by visiting the link at the top here that takes you to the webpage that has even more modifications that you can do.
This mower is a Honda self propelled lawn mower model HRU216 21″ cut