Rough idling and shaking from engine '87 450SEL

This weekend my mother was driving a long distance in her '86 Mercedes 450SEL and it started to run rough. When she pulled over there was a tiny bit of smoke wafting from under the air filter which sits on top
of the engine. The smoke stopped once the car cooled down, but the engine still shakes and idles more roughly than it used to. There is a noticeable clicking/ticking noise than gets louder and faster as the car approaches 2k rpm. There is a slight hesitation in acceleration. The oil pressure and temperature are consistent and normal. There are 194k on the original engine.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
450sel in '87?? don't you mean 420SEL

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Yes sorry 420SEL
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.