The Stovebolt Site Forums The Shop Area Engine Shop Why does engine run on after turning off?

My 69 Chev C10 250 wants to keep on running after I shut it off. It starts and runs great other then that. Is this a carb or distributor problem, timing? Any ideas on how to fix this?
Side note, my heat riser spring is broken. Any connection to the run-on?

The most common cause is the engine idleing to fast. Timing being a little to far advanced can contribute to the problem also. A fuel injected engine, when shut off, gets both, spark and fuel shut off, a carbureted engine only gets the spark shut off. If there is something hot enough in the combustion chamber to ignite fuel, such as a hot spark plug tip, or glowing carbon chunk, the engine continues to draw in fuel, causing the run on condition. Check your idle speed and timing first. If that doesn't do it you can dump some Sea Foam down the carb, with the hot engine at a fast idle. Be careful you don't want to hydro-lock the engine, just dribble it down the throat of the carb, shut it off then let it sit for a half hour or so. This should help de-carbon the tops of the pistons

can be either a situation where power is still getting to the coil, even though you think you've shut it off. this could be from a defective ignition switch or backfeeding through another circuit. it could also be "dieseling" due to ignition happening without a spark, perhaps from a hotspot in the cylinder - often caused by carbon deposits. you can stop this with higher octane fuel or cutting off the air.

try pulling out the choke (if its manual) after you turn off the ignition. this can often stop the dieseling.

Standard or automatic transmission? The primary cause of run-on is an idle speed a little too fast, causing the engine to Diesel. If lowering the idle doesn't help, try leaving the transmission in gear, and letting the clutch out just as you turn the switch off. It's hard for the engine to run when it's locked up tight to a transmission with the driver's foot on the brake.
Jerry

I tell this story because I often need to laugh at myself, and it keeps me humble. I doubt very much that my experience gives the correct explanation for your specific problem, but for what it's worth . . . here it is.

So there I was, out in my back yard, all by myself, trying to get my 1965 inline 230ci running for the first time in years. I connected the "battery" wire from my HEI distributor to the positive battery cable. Another Bolter told me that by doing so, I would not be able to turn off the engine using the ignition key, but would have to disconnect the HEI distributor wire from the battery to stop the flow of current to the coil inside the HEI. True, but I left it connected figuring that I could simply pull the battery cable off the battery post rather than go back inside the cab to turn the switch off.

I installed my new carb and ran the engine with an electric fuel pump. Once I was pleased with how it ran, I popped the positive cable off the battery post, BUT THE ENGINE KEPT RUNNING! I then disconnected the negative cable, BUT THE ENGINE KEPT RUNNING! I then disconnected the power to the electric fuel pump, BUT THE ENGINE KEPT RUNNING . . . . until the carb ran out of fuel! Lucky thing I was using an electric fuel pump and not the mechanical fuel pump mounted on the engine or it might still be running!

I laid awake trying to figure it out and then it came to me . . . "The alternator must still be good!" It must be generating enough current to keep the coil supplied with power to produce a good spark.

Laugh AT me or WITH me. I just hope you can imagine the panic that I had at that moment and laugh along.

I'm laughing with you, Lug.

Jay, I'm laughing WITH you. Over the years I've done some strange things....Like the time I drained the oil in my truck, went to the house and got something to drink ....came back out and backed the truck out of the shop and wondered why the oil light was on..duh!! (luckily no harm done. Drove it for years.)

Excellent statement-never seen it published before.
Speed more than anything, bring any older car up to 1000 rpm on a hot day and cut the switch without letting up on the gas and watch it trash like a headless chicken.

But now a days with unleaded gas you can almost wipe the little bit of carbon off of a 100,000 mile old engine's piston top with toilet paper. Its amazing how much of the old "carbon" (which literally had to be wire brushed or scraped off the pistons) was due to the lead in the old gasoline. Lead's residue was left all the way from the "carboned-up" combustion chambers and spark plug deposits to the light gray tailpipe condition seen after an extended highway trip. Years ago this was considered pure b.s but now everyone can see the proof.

Don't forget to check the timing, point gap, and plug gap as well.

Best wishes,

Les

Vacuum leaks can cause run-on also.

Great response, thanks everyone. First will check idle and any vacuum leak and see what happens. I might also try that "Sea Foam".

Just want to let you know, turning the idle down worked. Now, I will still check the engine timing...when I get a roundtoit!

My wife wanted me to let you know that she suggested that weeks before I asked here about it. There, I did it.

Thank for the feedback tonerman.That seems to work about 10 times out 0f 9.
Got that wife trained right.
If I ever get mine in tune with machinery like that I'll sometimes admit that she's right---until then???