First you need 2900 RPMs no load on yopur tach before you do anything. You need to makje sure that your tach is correct. If it is and you are only getting 2500 RPMs then you ned to check your linkage to make sure that you are hitting the throttle stop. The largest propeller your boat will turn under full load is that 21 X 19. The wheel you bough is much too big. You will over load the motor and blow something.