Small Engines (Lawn Mowers, etc.): Spark plug knock, kipor, leaded fuel

Question
Dear Ben,
I have a brand new Kipor KGE6500E home generator with the KG390 ohv gas engine. Unleaded fuel is specified.
It runs well but now and then, with no load, I can hear a very light knock. Only occasionally-----not continuous.
The manual says that there should be no knock, but if there is, to change the brand of fuel.
Does this make sense?
I was wondering if I should rather use high octane or regular leaded fuel to improve the burn.
I'd like your expert opinion Ben.
Your help would be sincerely appreciated.
With warm regards,
Laurence

Answer
Laurence
If the fuel is not of a octane of 89 or above it will cause premature ignition firing as the fuel is firing at the wrong time so the timing would be off, so that cause's the knock check you valve lash to make sure the valves are adjusted correctly and use a better grade of fuel.