My old YS 1.20 NC would give you fits!!! The low end adjusts with the regulator screw and by ear there is no way you can hear the RPM rise, it is running one second and goes lean the next click, you can turn the high end needle over a half a turn and hear nothing. I pulled out the tach and was finally able to see the rise and fall of the RPMs. This is the first one that I have ever had that is like that but it beat my mule. Runs beyond great once you find that sweet spot but finding it is a mystery without a good tach. It's my mystery engine!!