Guys, I am getting run on 50% of the time on shut down in my stock 326. I have hei ignition which I have tried setting between 12 and 16 degrees but to no avail. I bought a new 600 Holley which has made the car run better but no change, yes float levels and tuning have been set correctly. Disto is only about 6 months old is plugged to ported vacuum as I believe is correct I think I have read everything on the Internet but wonder if I am missing something? I read that when adjusting the timing you should remove vac line and plug at carb. I would not have thought this would make any difference as the port has no vacuum at idle anyway. An ideas I've just about pulled all my hair out!

number one cause of run on is too high idle speed. some cars have electric solenoid on idle screw that closes throttle when ignition turns off.

http://sohcsix.yuku.com/topic/2876/Idle-Stop-Solenoid found this on another site

Where you set your ignition timing has nothing to do with run-on, because ignition timing shuts off when ignition is shut off. Or at least it should...

When the engine is dieseling, is your "gen" light on? If yes, you often need to do "the diode trick" to prevent the problem...

http://www.msdignition.com/page.aspx?id=3296

This is not just an MSD problem...I linked it because they explain the fix better than anyone else with a nice diagram. But just like after MSD conversions, this problem is also common after HEI conversions...particularly if you have an external voltage regulator...which I assume you still do.

Carbon build up or insufficient octane can also cause this. Try Sea Foam.

A couple things come to mind.

hot spots. Are you running a really hot spark plug?

Try pulling the main power to the HEI and see if still does it.

Good point with trying a colder plug.

Thanks guys for your help, got the idle down as low as I can. Iam actually out of idle adjustment and it seems to have worked. Iam running about 14 deg of timing should I try to lower that to 12 and gain a little adjustment back on the carb? As I'm out of adjustment does that mean my jets are to big?

Jets do not come into play at idle on a Holley. Only the idle circuit.

Are you saying your idle set screw is all the way closed, and you're still too high? If so, you probably have a vacuum leak. Vacuum gage and tach should tell you something.

Have u tried higher octane gas? I put 87 octane in my 389 once in an emergency and I thought it was never going to stop running.