I have a 4-yr-old 46AX that has a lot of use on an Ultra-stik. It has just developed a power problem that shows up with a very narrow tuning range. My range on the high rpm adjustment needle is only about 1/4 turn total between rich or lean where the engine will even remain running much less running on the rich or lean side. I'm able to get about 11,800 rpm out of it. I've been running an 11x6 prop since I had the engine with no changes. In the air, the plane has noticeable less speed. When cold, turning the prop doesn't seem to have the same resistance as it used to. Any advice would be helpful.

Have you tried putting the low speed adjustment back to "normal" and tried from there?  I can't remember what the normal setting is, I believe you close it all the way and then open it 3/4 of a turn, but I'm probably wrong.

If the engine has had a lot of running, it 's possible it's starting to wear out. That means that the piston/cylinder fit is starting to become too loose. That will cause reduced RPM at full throttle, and an idle that is not as good. A worn engine will have little range for the needle from too lean to very rich. That's because you need a certain amount of fuel to make up for the loose fit...the oil in the fuel helps with the compression seal. A new piston/liner assembly should take care of things.