Ted;
That is a good idea. I test that theory out today. 

Thanks everyone!

  Ron Lane,  Meridian, ID

My vote is on the rotating fan is generating a current when the key is turned off.  You could potentially test this by making sure the fan is not turning when you turn off the ignition and see if the engine ‘run on’ problem goes away.

Lorena, Texas (South of Waco)

I didn’t think it would be carbon build up with you driving.😄. For intermittent vacuum leak returning to idle, how old is the carb? Maybe the throttle shaft? I’ve seen that even on new carbs.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Jeff;
Yes, the MSD box is connected to the tach. I have been having a problem with erratic idle and loosing spark at low RPM which is why I changed out the 10 year old MSD box with this new MSD Street Fire box which is supposed to provide good spark at low RPM. Before, I had to set the idle at 1,000 RPM or it would almost stall when I came to a stop. Even now it is not always steady at 800 RPM and and sometimes drops to 300-400 RPM. I'm wondering if it may be somehow vacuum related. 

  Ron Lane,  Meridian, ID

Is the tack driven by the new MSD box? Could the tack be off enough to indicate 800RPM at idle when the engine may be idling at 900? The slower the idle RPM the less chance of engine run-on after shut down.  Hope this helps, JEFF.......................


64F100 57FAIRLANE500

Thanks Guys! I do have a "Cooling Components" electric fan, but it does have a relay. It is not however thermostat controlled. I have it hooked up to a toggle switch which gets power from the battery side of the ignition switch. It's also possible thatI have been running on the rich side which could cause the carbon buildup in the cylinders. Maybe I need to take it out tonight and get on it a bit!

  Ron Lane,  Meridian, ID

Electric fans hooked up without a relay can backfeed the ignition.
Carbon in the cylinders can cause "hot spots" and ignite fuel in the cylinder. 

After checking out the MSD Troubleshooting link for "run on" issues, they indicate those problems to be linked to external regulators causing a small amount of current to back feed into the ignition circuit. Since I am running 1 wire alternator with no external regulator, I guess this isn't the problem.

  Ron Lane,  Meridian, ID

Mike;
It's the rattle/shake and then it quits. I will take a look at the MSD link for run on issues.

  Ron Lane,  Meridian, ID

You might want to read this on the MSD web page. Follow this link and click on the engine run on tab.  https://www.msdperformance.com/support/troubleshooting_techniques/