I switched from a 1996 75hp Mercury to a 1999 115hp Mercury, both 2 strokes. I kept the same prop, a 21". With the 75, I could get my boat up to about 30mph, with the 115, I only get it to 34 mph. My tach says I'm turning about 4800 rpm. If I go by the book, I'm supposed to drop to a 19" prop to get the rpm's up to 5200. My question is this, shouldn't I be getting alot better performance out of the bigger motor in the first place? It's got a great hole shot, but seems to putz out on the top end, leading me to think I should go up in prop size, but the tach says otherwise. I was looking for something in the 50mph range, not just 4mph difference. The 115 seems to run nice and smooth, in fact it's quieter than the 75 was, so I don't think there's anything wrong with it. I just don't get it... Any advice is greatly appreciated, I may try to check the fuel pump, filter, fuel ball, etc. tomorrow, but it all looks good visually. Looks almost new just opening the cowl..??????? Help!