Well adjusting it Engine is safe. However, this is one of those times where you need to stay out of the way from a piece of the Fan coming off.
For sure the Engine should be up to normal opeating temp.
You should not remain at the High rpms long; just enough to note what rpms start to shutdown the Engine.
Let it go back to idle and adjust the throttle stop screw and repeat the proceedure as needed.

If you ever have a Fuel Injection Pump Rebuilt you may have to fine tun it/readjust it because when they adjust it on the Test Stand there is no Engine to react to the adjstment.

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84 300D, 82 Volvo 244Gl Diesel

Is this sluggishness new. It could be the winter blend of kerosene in the deisel in your area. This will effect performance. The engine condition could have been marginal and the blend added to the problems

MO is my opinion. text lingo and NP is no problem. As for the for the car being sluggish it was always that way since I did the swap so I now know that the pump/linkage assembly was not adjusted properly because of all the pieces I replaced when I did the swap in combination with the linkage.

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Mad_Matt's Current Stable:
Oil Burners:
09 Jetta TDI The highway queen @ 105k
83 300D. Sold
Gassers:
16 A4 sport line
10 Passat
83 Recaro Trans AM WS6 Bone Stock Survivor
11 Harley Ultra