[SS MAN2]

It's a 1970 Chevelle with a 454. It has a factory type fuel pump (not electric). When I turn the ignition off it keeps on running for about 3 seconds and than shuts down. It seems to shut down slower than normal, not like before when it was instant. Any suggestions would be greatly appreciated.
Thanks, John

 

[B52bombardier1]

I doubt this is an electrical problem but I could be wrong. Back in the day, this was caused by hot spots in the combustion chambers from carbon deposits still hot enough to ignite fuel without a spark from the spark plugs. Poor quality fuel was the suspected cause.

So, please don't "ignite the night time petroleum" too much here trying to over think it or solve it. Simply buy higher quality (not necessarily higher octane) Top Tier fuels that have a good detergent additive package blended in - I'm a Shell, Chevron, Exon Mobil buyer whenever possible. I also periodically run a jug of Chevron Techron, BG 44 Platinum, AC Delco X66P or anything containing detergents from Berryman's in my tank at very high concentrations to keep my cylinders clean. Yes, call me weird here and I'm OK with that. Very high concentration = whole bottle poured in at a third of a tank of fuel remaining.

And please note - your combustion chambers did not get like this overnight and my offered solutions here won't work overnight either. This will take months of running these high concentrations of cleansers and good fuel through your engine to make progress but at least your are solving the problem passively and not tearing into your engine.

Rick

 

[meb]

It's a 1970 Chevelle with a 454. It has a factory type fuel pump (not electric). When I turn the ignition off it keeps on running for about 3 seconds and than shuts down. It seems to shut down slower than normal, not like before when it was instant. Any suggestions would be greatly appreciated.
Thanks, John

Last time ignition timing was checked?

 

[japete92]

If it basically 'stops running' but 'chugs' along for a bit, that's 'dieseling'. There is enough residual heat/compression in the combustion chamber to ignite the fuel w/o a spark. The simple causes are too low octane gas, or too much initial ignition timing advance, or both.

If it actually 'runs' (not chugging) there may be a problem with the ignition switch.

Pete

 

[oldcutlass]

This could be caused by too high of an idle speed.

 

[Leadhead]

Sounds like the timing is too far advanced to me. Try turning your distributor clockwise a couple degrees.

 

[427L88]

It ALWAYS happens to me if I have idle speed to high, as oldcutlass mentioned. First thing to check. idle speed, ot fast idle cam not coming off if so equipped. THEN to too much initial ignition timing.

 

[shovelrick]

Has anything been changed/adjusted recently?

 

[2/75NastyNewbs]

Ensure carb butterflies are not too far open.

 

[modern malibu]

If truly dieseling , as others have said the primary idle speed is set to high. If you cannot back the idle speed screw down enough to remedy the run on after key has been switched off, you will have to add an idle stop solenoid to allow the primaries to close more completely.

 

[Ry’s67138]

I had this exact condition. I turned down the idle speed just a few rpms and it solved the issue.

 

[David Ray]

IF the ignition timing is set correctly, initial and vacuum advance set up correctly, the carb throttle plates won't be open too far, and the idle transfer slots won't be feeding the increased quantity of air at idle.

Please let us know a few things, if possible:

Initial timing:

Vacuum advance timing at idle (full manifold vacuum port):

Idle speed RPM:

 

[DUTCH MAX HEADWORK]

Are you running electric fans?

What type of ignition system?

 

[SS MAN2]

Thank you for all the responses. Its not dieseling it continues to run normally, I do use 93 octane gas (Mobil /Exxon). No fans. I do use Seafoam every so often. There is a high idle. I let it warm up before I attempt to move it but the car is real cold blooded. I know there is a different cam in it has headers, double pumper carb, electronic ignition, bought that way so not sure exactly what I have. When starting cold, there is no choke, you have to run it, ( pressing gas pedal or it will stall) at about 1300 RPM, after about 5 minutes it will idle down to about 1100 rpm in gear. It's funny because at that point I am backing it out of the garage and it will want to stall. Once I back out of the garage and start going forward it runs fine. It seems very strong to me, sure keeps up with traffic. Very responsive with slight pedal pressure.
Thanks again, John

 

[DUTCH MAX HEADWORK]

Hook a test light to your coil positive or ignition hot wire, run the engine and shut it off and see if the power shuts off immediately when you turn the key off. This will rule out some backfeed electrical issues

 

[oldcutlass]

1100 rpm in gear means it's even a higher rpm @idle speed when placed in park or neutral. Yes, it will run on. Do the test in post #15.

 

[boltupal]

I,m going with Max Headwork. It could be the electric fans. Dis Connect them temporary.

 

[SS MAN2]

Thanks for all the help, I was lucky it was a simple solution thanks to all your suggestions, I lowered the idle and it stopped like normal.
Thanks again, John

 

[69SS454]

If it has an MSD ignition, there is a diode fix for that.

 

[Old greybeard]

My 350 Cutlass is now doing that, I adjusted dwell and idle, and it started to run on a bit. Was perfect previously. Waiting for a timing light, assuming adjusting dwell affected my timing. Find out this week.