I think I'm on to something
 

Hello everyone,
I have been trying to figure out why our '89 190e 2.6 seems sluggish and sometimes knocks bad during acceleration. Sometimes it is smooth and peppy. Could the timing not be advancing correctly? and how would I check it? Thanks to all, Have a good holiday tomorrow!

Not sure about the 6-cyl, but the 4-cyl 190E has easy timing adjustment by twisting the distributor (undo the allen-bolt). I can't figure why your performance should change at times, and the knocking is only occasional (unless you're changing fuel), could the allen-bolt be lose? Sounds unlikely but worth checking & tightening.

You should cure the knocking first. If you don't have the oscilloscope & equipment for this, timing adjustment is easily done by any half-competent tech. From here I guess you can then see if you still have poor performance.