[Natural Light Kid]

I know I'm about to open a huge can of worms. I bought one of the good old Mercury 40's that everyone claims runs anywhere form 40-99 MPH. I put it on a Southfork 16x46 flat bottom with a jackplate. Same set up with my old 25 Suzuki would run 30 +/-. With the 40 it's running 35+/-. According to everything I hear, it's hard to get those motors to run under 40 MPH on anything. I have
my jackplate set up so that I'm on the verge of blowing out so I don't think I can come up any higher. It is a stock motor with stock carbs. I had the carbs cleaned a couple of months ago when I bought the motor but could it be possible they are not in sync, adjusted right, etc.? I have a tach and I'm only turning up about 5200 RPM. The prop is stamped 14 pitch and does not appear to be altered.