Is he lifting off the throttle slightly as the engine starts to feel it is running out of revs?
From my understandng the rotax carb; Minimax's and Junior Max's uses are the same as the Senior carbs, but what happens is that, as it is a bowl/slide type carb which is more suited to the senior engine without a restrictor or power valve, too much air can enter the carb and not enough fuel (dont worry not enough to damage it!)
So by lifting off when the engine starts to feel its running out of steam, less air enters the carb and it picks up again and so more revs are acheived.
Again from memory this happened at about 10k revs.
When you net go out ask your son to give it go, dont worry it wont go away from him, one dab on the brake and it will stop accelerating!
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