1959 Runs after shutting off the engine

[PaulB]

After I've driven it for about 15 or 20 minutes and i turn off the ignition it coughs and sputters and runs for a second or two. I recently changed my automatic choke which the index was way over to the 2 o'clock position. It would take almost a minute of cranking to start and realized it was set for too lean a mixture. I think that happened when the rebuilt carb was installed. Anyway, i moved the index to the 12 o'clock position and now it starts right up after only about 4 or 5 seconds of cranking. Since its never done this before and thats the only change i made i wonder if you have any thoughts what might be causing this?

 

[Oly Bob R]

Check the timing and try a different grade of gasoline

 

[Ward 57]

It's called Dieseling. Running like a diesel which uses no spark. It's usually from carbon build up and too low of octane gas. it was not uncommon for cars built in the '50s &'60s especially if only used for short trips. a few long drives should help. Your car will like it.
There are carbon removing liquids you pour down the carb while running at high idle. Just beware they cause huge plumes of white smoke out the exhaust.

 

[74 Harley]

Definitely check your timing, carbon buildup, poor gas , even just running a little hot can cause this.

 

[312YBlock]

After I've driven it for about 15 or 20 minutes and i turn off the ignition it coughs and sputters and runs for a second or two. I recently changed my automatic choke which the index was way over to the 2 o'clock position. It would take almost a minute of cranking to start and realized it was set for too lean a mixture. I think that happened when the rebuilt carb was installed. Anyway, i moved the index to the 12 o'clock position and now it starts right up after only about 4 or 5 seconds of cranking. Since it’s never done this before and thats the only change i made i wonder if you have any thoughts what might be causing this?

Unfortunately due to the cylinder head design, and today’s fuel there really is no cure for it. I feed my 312 Sunoco 91 octane, with 1 ounce of SeaFoam gas treatment per gallon. The sea foam will clean the carbon out of the cylinders and valves, it’s amazing stuff. Carbon will form hot spots and contribute to engine run on. If you have a stick put it in first with your foot on the brake, lift the clutch till you just feel it and turn off the engine if Auto shut it off in gear. Engine run on can cause serious damage to the engine. Also use Autolite #45 plugs gapped .035, it’s colder than a 46 and will help lower cylinder head temperature.

 

[evensteven]

I used to watch my old mechanic, run the engine at a high idle and squirt water from a hose directly into the carb off and on. It would cause a violent explosion, and he would blow the carbon out the tailpipe. I had never seen this before, but he swears by the procedure.

 

[74 Harley]

Forgot to mention make sure your idle isn't set too high.

 

[Ward 57]

I used to watch my old mechanic, run the engine at a high idle and squirt water from a hose directly into the carb off and on. It would cause a violent explosion, and he would blow the carbon out the tailpipe. I had never seen this before, but he swears by the procedure.

Well, they didn't have the chemicals we do now. Only side effect is that it would increase the compression and possibly break a ring or gasket.

 

[312YBlock]

Unfortunately due to the cylinder head design, and today’s fuel there really is no cure for it. I feed my 312 Sunoco 91 octane, with 1 ounce of SeaFoam gas treatment per gallon. The sea foam will clean the carbon out of the cylinders and valves, it’s amazing stuff. Carbon will form hot spots and contribute to engine run on. If you have a stick put it in first with your foot on the brake, lift the clutch till you just feel it and turn off the engine if Auto shut it off in gear. Engine run on can cause serious damage to the engine.

 

[312YBlock]

Unfortunately due to the cylinder head design, and today’s fuel there really is no cure for it. I feed my 312 Sunoco 91 octane, with 1 ounce of SeaFoam gas treatment per gallon. The sea foam will clean the carbon out of the cylinders and valves, it’s amazing stuff. Carbon will form hot spots and contribute to engine run on. If you have a stick put it in first with your foot on the brake, lift the clutch till you just feel it and turn off the engine if Auto shut it off in gear. Engine run on can cause serious damage to the engine. Also check the heat range of your spark plug, you want the coldest plug you can use for your engine, it will help lower the cylinder head temperature.

 

[Brax]

I used to watch my old mechanic, run the engine at a high idle and squirt water from a hose directly into the carb off and on. It would cause a violent explosion, and he would blow the carbon out the tailpipe. I had never seen this before, but he swears by the procedure.

Yes best way to do it 3500 rpm use a spray bottle and do a little at a time 10 minutes later carbon be gone. Use to add water injection to old cars works great

 

[Oly Bob R]

I always used “Gum Out “ putting into carb at a fast ideal with the wind on my back and my neighbor is not home

 

[Ward 57]

Yes best way to do it 3500 rpm use a spray bottle and do a little at a time 10 minutes later carbon be gone. Use to add water injection to old cars works great

I tried that water injection on my '66 running up a hill and broke a compression ring land.

 

[Brax]

I tried that water injection on my '66 running up a hill and broke a compression ring land.

To big a jet used , I used WI on 312 Tbird and 429 SCJ drag pack Torino, and 427 fairlane never any problems except horse power,

 

[rapidrobert]

After I've driven it for about 15 or 20 minutes and i turn off the ignition it coughs and sputters and runs for a second or two. I recently changed my automatic choke which the index was way over to the 2 o'clock position. It would take almost a minute of cranking to start and realized it was set for too lean a mixture. I think that happened when the rebuilt carb was installed. Anyway, i moved the index to the 12 o'clock position and now it starts right up after only about 4 or 5 seconds of cranking. Since its never done this before and thats the only change i made i wonder if you have any thoughts what might be causing this?

Sounds like the idle is set to high.

 

[nieuwr]

I had the same problem on my '65 with 390 engine. Well, maybe worse, as it dieseled for several seconds before stopping.
In hindsight, I think it was caused by carbon buildup from running too many times for only a few minutes at a time when I was trying to get the choke set.
I used Techron, which worked just great. It has a high % of PEA (Polyether Amine), which seems to be the stuff that loosens or dissolves the carbon.
However, the dieseling started to come back. The mechanic at a local classic car restoration shop told me the problem was that my timing was too low. How can that be when I set it to factory spec (6 BTDC)? He said the fuel we use now is much different from what was used back then. Increase the timing until it is happy. Mine is now around 25 BTDC and happy. Starts, runs, stops in command. No more dieseling.
FWIW.

 

[74 Harley]

@nieuwr, if your engine is running happy at 25 degrees advance then either you need a timing chain in a bad way or your harmonic balancer has slipped. (And needs replacement)

 

[Tom Alpers]

I had the same problem on my '65 with 390 engine. Well, maybe worse, as it dieseled for several seconds before stopping.
In hindsight, I think it was caused by carbon buildup from running too many times for only a few minutes at a time when I was trying to get the choke set.
I used Techron, which worked just great. It has a high % of PEA (Polyether Amine), which seems to be the stuff that loosens or dissolves the carbon.
However, the dieseling started to come back. The mechanic at a local classic car restoration shop told me the problem was that my timing was too low. How can that be when I set it to factory spec (6 BTDC)? He said the fuel we use now is much different from what was used back then. Increase the timing until it is happy. Mine is now around 25 BTDC and happy. Starts, runs, stops in command. No more dieseling.
FWIW.

Dieseling has nearly nothing to do with timing. The only connection to timing is if the timing is so far off that the heads are running way hotter than normal that extra heat that is still there when the car is turned off can cause hotspots and dieseling. Other than that, when the car is off timing makes no difference because the plugs are not firing. You could yank all the plug wires off and it would keep dieseling.