mmmm, its not really much info youve given us to work with.... youre saying that the engine clunks when you put it under load and run a hi gear.
its a petrol engine, and its small...its not a diesel. they prefer to run at 5grand or more, not a little bit above idle.
from what you described as being the problem, we replied with what we feel is causing the noise, and how to cure it. not being smart, not giving you shit, just stating an opinion.
if you dont like the opinion, think about rephrasing your question so it doesnt make it sound like its an "operator error", before getting your knickers in a knot and blaming all and sundry...its immature.
its a chinese motor, and its cheap....rev the goddamn shit out of it...
changed the oil frequently? like with every tank until maybe five tanks have been run through it? or still on the original shitty cheap chinese oil that would now be resembling whata i cough up in the morning???
?
change to a 20/50w and the noise might go away...as well as change to 2nd or third...