If your running stock compression, Lycoming spec. is 25 deg. advanced. I set mine with a timing light. Mark the flywheel at 25, and at about 800 rpm's you can get it on the money. worn timing gears or the rubber bushings will cause the time to be off, if you set at the fire out hole in the side of the mag. If you bump the timing to 26,7,8..it will make the cylinders run hotter.