Hi everyone, thanks to the comments on my last post and some digging through various forums, I've solved my low power issue on my 85 300SD. It's now making good power but I have a very minor issue to solve. When I push the pedal all the way down, my throttle linkage still has about a quarter of an inch of travel left. I'm assuming this is down to an issue with the linkage attached to the firewall, I've adjusted the linkages on the valve cover so that https://snaptube.cam/ I can actuate it through its full travel with no binding or play. I tried unscrewing the ball socket at the end of the linkage that comes up from the bottom of the firewall but it unthreads completely before raising up the socket enough to fully push the ball it's attached to. I've attached a picture of roughly the amount of travel I have, and I can take some more pictures tomorrow.

What is the condition of your engine mounts? A collapsed mount on the LH side will allow the engine to drop/sag and creates a longer throw for the linkage.

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Oh man, those stupid overly-complicated linkages can be a pain. I chased mine all over the place for years and finally figured out by accident that someone previously had put the linkage that goes around the sway bar on backwards. Now, one of these days, I'm going to have to go over the whole thing again and hopefully get it working right finally.

Lots of things can affect them, and you may find that setting everything to the "FSM measurements" doesn't work either because of wear throwing it off.

You just have to take each piece one at a time. And yes, definitely check seemingly unrelated things like the engine mounts. There's also the firewall pivot to look at, and the pedal itself, which can get wonky.