1979 Merc 40hp lack of top end....

Hi all, I have a 1979 Merc 40hp remote elec.start (I think model 402?). It's mounted on a 1982 17' Alumacraft. The motor and boat were sitting for about 5yrs before I got it. Since, I have put in a new stator, new starter relay, new water pump, new fuel tank, carb. cleaned, and fuel screen cleaned out. Its been to a marine dealer for a "tune-up". The motor starts good, has nice pick up but seems to die off from 3/4 to top end. It runs smooth until 3/4 as well then starts to "jump" a little bit then bog down then pick back up. I'm only topping out @ 16mph as well. What could cause the semi rough running @ hi throtle and is 16mph all i'm going to get out of this thing? This is my first boat so i'm rather inexperienced with outboards so any advice would be helpful.
Also, does anyone know where i can get a tach to hook up to the controls?