Do you mean you have never touched the low speed, or you don't very often? The low speed needle needs attention once in awhile also. This probably isn't your problem but I have had two different engines from two different manufactures that the needles would adjust themselves. one did it every 2 or three flights and the other did it every flight. I fixed those with a small piece of fuel tubing around the needle. Whenever I find any engine running odd I always check both needles before I do anything. If that doesn't help I try different glow plugs. I am sure you already know all of this but I find when I have a particular engine that runs really good for a long time I tend to forget about the basics.-BW