You are correct, there is very little published data about the metering rod adjustment.
The metering rods do have a screw-type adjustment to them. I have some examples in my garage but either its too dang cold to go out there or my wife has other plans for me at the time.
These pictures are ones I swiped off the internet. The metering rod adjustment is something that a few people mess with and it causes all kind of problems. I think the newer TQ’s, they took that adjustment away.
The rod (white arrow) is also the screw which extends to base of carburetor. The screw (yellow arrow) sometimes has a cover that needs removed to adjust it (depending on year unit was made).
The official adjustment is: Depress the metering rod piston until it lightly bottoms in the bore and hold there. With a small screwdriver, turn the screw counter-clockwise until the primary piston stops going down. Then, slowly turn the screw clockwise noticing when the piston begins to move up. When it begins to move up, turn the screw 1-1/2 turns clockwise. This is a starting point. From there you can fine tune it by turning the screw clockwise to richen the mixture and counter-clockwise to lean it.
This lever/plate is what pushes the metering rod piston up when the accelerator petal is pushed down, which richens the fuel mixture (as well as the accelerator pump).
I have seen a few TQ’s where the “step up actuating lever” was bent into a “U” because they turned the screw too far to richen it up (it kept the metering rods full up, all the time).
BudW