81 280SL Idle Problem Or...?

[WINTS]

A little background to start...The 280 has had a vacuum leak since I first bought it, but always started when cold, sometimes after a couple of turns, and always when warm. The idle was around 900-1000rpm. It ran well however, with no hesitancy, or missing.

I recently took it into my shop to get the valves adjusted. That was done, and while there the coil had an intermittant grounding problem that my mechanic said he fixed. The coil was not replaced.

The car drove fine the first 20 miles, but now, some 80 or so miles later has been difficult to start, and has started stalling at idle after about 20 seconds. It will restart after a few tries, but there is hesitancy under 1500 rpm all the time, although above that it seems fine.

As it stalled out on me today, I tried to gently catch the gas to prevent it, but heard a quick sudden rattle, and then a puff of smoke from the coil/air filter area under the hood as it died, with a burning smell. That had me worried...

It started up again fine and I drove home real carefully.

Thanks in advance for any replies...