Timing chain slop
 

Well, just so everyone knows, I did use the search function...

Here goes,

I adjusted the valves on my 84 300SD on Saturday to eliminate the smoke and power loss issues. Success!!!

Problem... I still have a rough idle. Well, looking back, I think my inexperience on these engines might be showing itself. What remember seeing was that the timing chain seemed loose. I did not think much of it at the time, but I could move it about 1/2 an inch to either side. I think that this might explain some of the noise I am hearing from under the hood, and also that wierd "rough spot" while under mild deceleration.

What think ye? Timing chain replacement?

BTW, I am using the DIY procedures from this link. http://dieselgiant.com

Just speculating
 

You might need a new chain, do you know how many miles are on the engine? But I would think the looseness on the chain that you see is caused more by a bad tensioner then a worn out chain. Better keep checking and I am sure you will get many good ideas from the people here.

Check the cam timing using the 2mm lift method.
Check injection timing using the drip tube.
Pull you injectors have them poptested and inspect your prechambers.