Just got a 15' fiberglass skiff that has a 84' Johnson 70 on a jackplate running a 13.5x17 prop. I installed a tiny tach on the motor and now when I am running WOT the rpms were getting up to 8k and the boat wants to idle at ~1500-1700 rpms in gear. I have messed with the idle screw and if the rpms drop lower at idle it just cuts off. From what I can gather the motor might be running a bit lean causing the bad idle. Do I also need to run a higher pitched prop to cut down the rpms? The boat also wants to porpoise sometimes, can this be compensated by raising the motor on the jack plate some? Thanks and let me know if you need more information.