That steady reading you’re struggling to get is possibly to do with limited ignition coils. Many modern 2 coils have built in circuitry to adjust / advance and limit spark. When the limiter kicks in, unless you have a tachometer that can read limited coils, you’ll get gibberish - it will jump around with all different numbers and won’t stay steady.
My advise is to not buy an expensive edt9 - it’s not very good. Instead, use your tach knowing the second the numbers start jumping around you’ve hit the limiter. You can set stock engines there and they’ll be safe. The manual specs will tell you the max rpm is “rev limited ….” Set it and forget unless you want to dive into the subtleties of tuning.
