I have a 1969 390ci and it wants to keep running for a few seconds after key is turned off (ive seen it called dieseling). I have always thought it this was a timing issue? Im not having much luck searching online? I pulled #1 plug and it was dry and tan in color, which im seeing means to hot or to lean? I m going to check the timing, then R&R plugs,wires,distcap/rotor this week. It has a new carb the PO installed. Do you think it is a fuel/mix issue? If so What is best way to set? Ive found alot of videos showing idle mix setting could that be a prob idles fine @ around 8-900rpm. i hate to put new plugs in a ruin them if its a mix issue.
Thanks

 

Gas engine dieseling:
--deposits or imperfections in combustion chamber, on plug or on piston remain hot enough to ignite incoming A/F charge. Plugs too hot might be an issue. Tan plugs in good condition should be fine.
--idle set too high, allows throttle to be open enough after shut down to pass enough A/F to support combustion.
--carb not settling down to throttle closed position for some reason other than idle set screw.

How did you set your idle? A combination of idle set screw, the idle bleed valves and timing will give you a smooth fairly low idle with the throttle plates closed enough to get below the idle circuit holes you should be able to see in the throttle bores.

Is this a pickup 390 with low, low low compression or a car transplant with 10.5:1?

Does it have a dash pot or high idle device for air conditioning idle adjustment?

https://www.championautoparts.com/Pa...ark-plugs.html

https://cimg1.ibsrv.net/gimg/www.for...57d3189a92.jpg
From:
https://www.ford-trucks.com/forums/1...t-install.html

https://www.ford-trucks.com/forums/2...on-2150-a.html

 

I would go with too high of a idle if set to 800 - 900 RPM
That could also be to high to set the idle mix screws as the throttle blades are open a little.

Get a vacuum gauge hook it to manifold vacuum for adjusting.
Get motor up to normal temp when doing any adjustments.
Drop the idle down to to 600 RPM. Now adjust the mix screws each one has to be adjusted the evenly number of turns.
Adjust one screw a little (in or out) to get the vacuum to the highest reading noting the number of turns.
Now adjust the other to get the highest reading (in or out) now if you find it did nothing turn the first screw in half the number of turns and see if the 2nd screw will now make a change.

When doing this the idle may raise lower it back down to 600 RPM and readjust the screws for the highest vacuum reading.
Once you have the carb adjusted put it in gear and see if it will stay running. If it stalls up the idle speed to 700 RPM and try again. I would not go over 800 RPM for a idle or you can get the dieseling.

If you check the timing and adjust it you will need to go back and reset the idle speed to what it was before the timing adjusting.

As posted above it you have a dash pot or a solenoid kicker for AC or maybe not you set the idle speed by it and with no power to it the idle should drop below the setting lowering the idle speed so no dieseling.
Dave ----

 

Yes it does have the dash pot for ac, i backed the idle screw off but it didnt affect the idle speed? Its a cheap no name carb on it, idle was sticking at 1200 tap on pedal dropped to 900 sat at idle for a few mins and dropped to around 700. Kept doing that all day? like throttle is sticking a little? but anyway i got a can of seafoam intake cleaner, sprayed it down the carb and thrashed it up and down the road, it shut off fine. Hopefully it just needed carbon cleaned. Ill prob do my tune up tomorrow