Mercury 15hp 4 stroke, knock at WOT

[Valv]

I have a '99 Merc 15hp 4 stroke which started to knock at WOT. I toke it to a marine shop and they rebuilt carbs and reduced speed from 5900rpm (too fast) to 5500rpm.<br /><br />I was talking to some people and they mentioned Mercury had some problems with knocking 4 strokes. Is that true ?<br /><br />Anybody else had problems ?<br /><br />Thanks.<br /><br />Val