'04,150 ox66 tach was ok last week. Yesterday, I noticed the tach was pointing at 1500 rpm at idle but I know from the sound of the engine, the rpm is much lower. I'm using 19p prop and my max rpm on it is 5500 rpm, now shows around 6500 to 7000. The engine runs fine and I kept the engine rev low. I'd like to know if anyone can provide information as to why and maybe some suggestion. Thank you.