Engine "Run On" after Shutting Down

[markofsa@me.com]

My '71 280SL (with US emissions) engine continues to run for about 2-3 seconds after I turn off the ignition.  In this part of the county we use Super Unleaded 93 ROM octane instead of the 98 octane the car was designed for.  The timing is set at 8 degrees after TDC according to the book at 800 RPM's with vacuum. The ignition has been converted to a Crane optical ignition and works beautifully.  The advance and all of the emissions stuff works as designed.  The temperature gauge holds at 180 degrees Centigrade.  In the past the car seems to have been "set up" in a way that caused carbon deposits to maybe form.  The old spark plugs where covered with black soot that looked like black velvet.

Any thoughts on what is causing this "run on" and how to approach the problem?

[ja17]

Hello,

Some times a good high rpm exercise session (Italian tune-up) will do the trick! Drive the car in a lower gear and spend some serious time in the 3500 to 5000 rpm range!

[jacovdw]

What type of spark plugs do you use?

[markofsa@me.com]

I just put a can of BG 44K into the gas tank and went for one of those "Italian tune-up" and that did the trick.  Apparently, there were carbon deposits in the combustion chamber what was causing the run on problem.  I intend to use the this product or something similiar from time to time in the future to eliminate the problem in the future.

The timing has been corrected to return it to factory specs so I think this problem will be minimized going forward.

I'll put a recommendation in for the BG 44K product.  I had never used it before, but other posts and local knowledgeable recommendations prompted me to give it a shot.  I can say that as the drive progressed I perceived an increase in performance and the engine really smoothed out.  It's not cheep ($18.50 US per can) but it seems to do the job.

Thanks for all of the great ideas that helped me come up with a plan of action.