I have a 1985 evinrude 150hp on a 20ft wa with a 14 1/2x 19 prop. The boat weight is about 3200lbs with motor. This has just had a ring job and carb rebuilt this year when I bought it. However I'm topping out at 4200 rpm. Any ideas how to get this up to around 5500 rpm. Other than this the motor...