Just a word of caution, I have a 150 EFI and my partner has a 150 XR6. The max rpm on both of these motors is 5600. I'd assume yours to be the same and if so you are over revving your motor. From what I have gathered these 2.5 liter mercs dont have a rev limiter so there's nothing to warn you that you are over revving till it blows. Sounds to me like you need a prop with more pitch, or yours is slipping badly.