Hi Brett:
I will bet that the sound that you hear on the Lawnboy changing rpm is due to the action of the governor! I have never heard a Lawnboy run in a four cycle. Perhaps I did not recognize the four cycle sound.
As I type this, I am rebuilding a 5269 and the next project is short blocking a 5029 Brickhead. I just can not say no to a D engine. To me they are the greatest mower ever sold. My wife trims with a 19” 5239 and loves it!
In any case, stay well,
Frank McCune