PT20J

Try adjusting the height about halfway between high and low and sitting on it and shifting your weight to see if you can get it to slide forward and off the rails so you can start over.

There's a guy in my neck of the woods that does dry ice blasting. Might be interesting to try. No residue.

The M20J IPC calls out the Cleveland part. Flat covers may have been field fabricated.

It’s a Cleveland part. https://www.aircraftspruce.com/catalog/lgpages/wheelcovers2.php

When I used to volunteer at a museum that had a lot of WW II vintage airplanes we ran into this all the time. Scotchbrite and elbow grease followed by a coat of epoxy primer was how we dealt with it.

Those brackets are just elevator and rudder stops. They aren’t structural. Just clean them up as described above.

75 deg F LOP is pretty lean. Keep in mind that we are dealing with last century mechanical technology here. I sometimes notice a lag in FF with mixture changes. I have a spot around 9.9 gph where it seems to hang with no change in FF even though I can see the EGTs rising as I lean the mixture. Try running the leanest cylinder no leaner than 50 deg F LOP and see if that solves it.

The Hartzell prop hub is not supposed to be filled with grease. Hartzell has specific recommendations for greasing but if the hub is over greased the only way to rectify the situation is to disassemble the prop and clean it out. https://hartzellprop.com/MANUALS/131-0000-A.pdf

Usually sticking valves show up when cold on start up. But I've had them stick momentarily in flight. I've also had an IO-360 miss a beat when running LOP. There is a lot of cycle-to-cycle variation in combustion and this is exacerbated with lean (slow burning) mixtures. In fact cycle-to-cycle variation is cause of the roughness you feel when you lean to the limit. So, everything being a bell shaped curve, if running very LOP, there will be an occasional combustion event out on the tail of the curve that will reduce power from that cylinder momentarily. The fact that it doesn't show up on the EGT trace indicates that it was likely just a random transient event. If it keeps doing it on rich mixtures, a wobble test is the definitive test of valve stem-guide clearance.

3 puts TDC at about 4 o’clock on the low blade viewed from the rear with a two bladed prop

Can you see an EGT drop when this happens to figure out which cylinder? Could be ignition, fuel injector nozzle, or sticking valve. I’d eliminate the easy stuff first: clean the nozzles and spark plugs.

The OP paid a not insignificant amount for an inspection. Accordingly, the results of that inspection are his property and have tangible value to anyone interest in purchasing the airplane. It is not unreasonable to offer that information to interested parties for a fee. It shouldn’t concern any of us uninterested in purchasing the airplane.

I’ve owned my M20J for 7 years. I’ve bought a bunch of parts, but the only Mooney factory parts were 3” rudder pedal extensions and a nose gear leg.

It’s common for those LORD panel shock mounts to break. They are only there to reduce vibration on panel-mounted gyros. If you go glass, it’s best to replace them with solid spacers. There have been some reports of issues with Garmin ADAHRS if the panel is shock mounted.

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Lost motion due to wear in the steering linkages will make precise steering more difficult because it introduces a dead zone between left and right pedal inputs. This is most noticeable at taxi speeds. Failure to track straight at higher speeds is a caster angle issue and that’s what the service bulletin addresses.

Try deep cycling the prop all the way to the high pitch stop a few times. That worked for my McCauley. Couldn’t hurt to try on your Hartzell.

I just noticed that you have a M and this drawing is for a J, K, L. I would think the L would cover the long bodies, but Mooney has a habit of changing stuff, so best check it.

If you cannot find new or used, you could have some made as an owner produced part. Rudder Extension Drawing 720115.pdf

That’s way too loose. Here are the limits from the M20J S&MM.

A clue to the purpose of the bungees is that Mooney calls them "trim assist bungees." If you rotate the trim wheel from stop to stop you will notice two things: First, because of the arrangement Rob @takair noted, the springs will move the elevator in a direction that aids the aerodynamic effect that the stabilizer is trying to achieve. The second thing to notice is that it takes a lot of turns of the trim wheel to get from stop to stop. If the stabilizer incidence change alone had to remove all the stick force, the stabilizer would have to move over a larger range and this would require even more rotations of the trim wheel unless the gearing was changed which would reduce the mechanical advantage and make the wheel harder to rotate. But by using the spring arrangement to assist the stabilizer, less stabilizer movement is needed to achieve the same stick force reduction. '

The M20J IPC doesn’t appear to list any 120068 part numbers. The M20J wing root fairins are all 120001. Frank Crawford at the factory would know if there is any difference.

The idea behind any trim system is to remove stick force and this can be done in several ways, or by a combination of methods. The most common method is a trim tab that replaces the stick force necessary to hold a particular elevator position with an aerodynamic force. Another way is to allow the incidence angle of the stabilizer to be changed in flight. A third method is to have variable force bungees in the control system so that the bungees can provide the force necessary relieving the pressure on the stick. Mooney chose a combination of variable stabilizer incidence and bungees.

In engineering there is an old saying that there is no such thing as a single change. I believe the short bodies trim out with the elevator aligned with the stabilizer. However lengthening the fuselage for the mid bodies threw things off and they trim in flight with the elevator slightly trailing edge down. I was told years ago by an engineer at Mooney that Lopresti looked into this when modifying the M20F to make the M20J and decided that fixing it would require significant rework to the empennage to change the angle of incidence of the horizontal stabilizer and the drag was minimal and not worth the cost to fix it.

Are they the same part number?