I have a 17 foot stratos boat with a 90 johnson that has 140 heads and carbs on it.It replaced a 140 that went bad.Now my problem , its doing the same thing the 140 did.Only turns 4700 rpms and 42 mph , thats max!It does have a jack plate on it and is set about 1/2 to 3/4 inch from bottom of...