My Mooney Ovation with KI-256 and KFC225 autopilot has an issue where it hunts up and down a couple of degrees. It is really annoying. All other functions work, VS, ALT capture, etc. It was varying up about 4 degrees and down not as much, barely perceptible altitude change about 20 feet. I took it to a highly respected autopilot shop, recommended from here and was not impressed with results. They did some work on a pitch trim servo, tested the ADI, tested and blew out static system. They seemed highly confident in each fix, in order I listed but they finally tightened up a spring in the elevator control system and told me it was fixed. I don't believe it is. I fly the plane for long legs, hours at a time and the problem still exists, I think tightening the spring just dampened out the oscillations.
More details:
The trim doesn't seem to be running during porpoising
Same porpoising when left in PIT mode only, not ALT
I think it is also porpoising in VS mode but hard to tell
I spent $3600 on them testing and replacing parts when the "final solution" was tightening a spring. I do have a call into their expert, but he hasn't called back yet. Is this level of fluctuation just the way it is? I find that hard to believe. I am just trying to educate myself so when I get a return call I can question intelligently.
Any feedback appreciated