Why does Honda z50r kick into gear from neutral some of the time?

I've had a 94 z50r since I was 10 or so and as far as I can remember it would kick out of neutral and into 1 or 2 gear never 3 some times. It ussialy does it after I start it up, the engine rpm slightly builds up by itself and all the sudden it kicks into gear and tries to fly off… My friend was going to buy it from me until I told him about it and now he won't take it becaus his michanic said the clutch is busted (when it's completely word of mouth) or something and it's a 300 reapair. Can sombody explain the problem for me? It's never bin a problem, it just does it sometimes

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