Diagnosis by Smell

[Finbar]

All five senses are often required to understand what the Rover is trying to tell you. The color or smell of a fluid, and I've heard some old timers say even the taste, can lead you to the source of an issue.

My 1981 right hand drive series III with a 2.25 L 4 cylinder petrol engine is running like a dream. However, after pushing it hard on the road to 50 MPH or above for any length of time I find that it tends to run-on after I've stopped and turned off the ignition. I assume there is carbon buildup inside the cylinders that is just getting hot enough to cause ignition of whatever fumes are left when I turn off the engine. The part I don't understand and would appreciate some enlightenment on is the smell I smell when the engine finally grunts itself to a stop.

It doesn't smell like gas or oil (of any kind). It is more pungent. IT is particularly strong if I pull out the choke to 'quite' the engine when it starts running-on.

Any ideas what this might be? Am I wrong about the carbon build-up? I've tried sea foam in the carburetor and the gas tank but it hasn't made much of a difference. I've also noticed a small about of dark, oily sludge at the base of my Weber carburetor. I don't think this is related but thought I'd mention it just in case.

Thanks in advance for any feedback.

[SafeAirOne]

• Dark, oily sludge at bottom outside of carb usually = external fuel leak.
  
• I think you're on the right track, hotspot-wise. Just need to figure out the cause/cure.
  
• Since the unintentional ignition during engine run-on isn't timed with the valves, often the engine will run backwards while dieseling, sending exhaust gasses back up through the engine air intake system. Without knowing for sure, I feel safe saying that the acrid fuel/air/exhaust gasses bubbling backwards through the oil bath air filter has a tendency to stink.
  
  That's my theory, anyhow...

[80sailor]

Not to hijack, but what is usually the cause of dieseling?