I'm having an argument on another forum. A fellow shuts his car off (older 50s gas model) and it continues to run or diesel. He gets all these suggestions about carb settings, timing issues, etc. My opinion this has nothing to do with it...when key is off there is no spark, timing has nothing to do with it. My suggestion was simple: turn key off, pull plug wire, if it continues to run you have eliminated spark. It has to be hot spots (carbon) in combustion chamber. Try water. I used to rev motor about 3-4,000 rpm, slowly pour little water down carb, choke wide open. Don't let it die...just sputter, stumble...rev back up. Usually carbon breaks loose with steam, all that crud comes out tailpipe. If it continues, and plugs are correct heat range, you'd have to pull the head (s), clean everything. Igniting timing has nothing to do with run on. Am I right? Sent from my SM-S320VL using Tapatalk
Nope Bad timing Bad tune up Could be carbon or fuel, happened to me on a ford 460 and a few sbc’s always ended up being tune ups required.
If it is dieseling you have air and fuel getting to the cylinders with an uncontrolled heat source to ignite it since the key has shut off the controlled heat source (spark). Possible heat sources: High compression ratio, incorrect ignition timing, lean air/fuel ratio, carbon deposits (burning oil or incomplete combustion), inadequate cooling system, poor octane fuel. Does he have a big lumpy cam that needs a high idle speed to keep it idling. This will allow it to draw air/fuel into the next intake stroke every time a cylinder fires because the throttle blades will not close enough to adequately shut down the air flow. But you still have an uncontrolled heat source. If this is the case and it's an automatic, try leaving it gear when it is shut it off. (A crutch, not a cure.) In the early days of emission controls GM used an idle speed solenoid to close the throttle blades completely on engine shut down to prevent dieseling..
If it is a stocker and the owner doesn't get out on the highway and run at highway speeds for a length of time on a regular basis carbon deposits on top of the pistons is a common cause. The "It's old so I keep it under 50 and don't drive on the freeway at all and mostly keep it under 35 nonsense. I used to have to take my grandfather's Dodge out about once every two months and run a quart of atf dribbled though the carb (Rick who owns all the rusty tin 3/4 of a mile south of me will testify to that as I poured the trans fluid in across the road from his house) then take off down 97 and up the grade until it cleared out and ran like it should with a few miles at 75 or 80 to make sure it was running right. Cruise back to my grandfathers where I live now and park it. The man never broke a speed limit limit in 60 + years of driving and usually drove ten under. I have to agree with the idling too fast as I am fighting a 300 six that does that now and diesels if it is hot. Can't say it any better than X-cpe did above though That pretty well nails it.
Dieseling is a result of very high combustion temperatures and fuel getting into the cylinders. It does necessarily mean bad tune, certain engine combinations will produce the dieseling effect. It usually is worse in higher temperatures, both thermometer and engine temp. Oem’s recognized this back in the day and did all kinds of things to stop it. Ford, among others had a throttle positioner that would close the throttle plates completely to stop dieseling. Pontiac and other GMs used to turn on the A/c for a period of time to load the engine to stop dieseling. On my OT DD it had a tendency to Diesel, I alway killed it in gear and let the clutch out at the same time. Sometimes you just have to file it under shit happens. But carbon, high idle speed, fast timing, cheap fuel, high temperatures all contribute to dieseling. Sometimes it’s a combination of things. Bones
Been battling dieseling for 30 + years. I just turn the car off in the Drive position when it acts up.
You don't. You drive or let your car idle until it diesels. You will know because you can't shut if off, it keeps running. You open your hood and remove your air cleaner. Have someone operate the throttle while you pour water down the carburetor. They should be under the hood with you so you can work together on this process. Don't let id die or it will be a bitch to get started again. I prefer cold water because when it hit the hot, glowing carbon in the combustion chamber the carbon bit should break off and go out the exhaust. I agree with Mr48chev about taking the car/truck out on the open road and put the pedal to the metal for a few miles. Either approach or both will fix your problem. This worked back in the early 60's.
I agree with all things said here, even a well respected local engine builder/racer friend said basically the same thing...an out of tune engine can get "all coked up" then dieseling. My point was timing off in and of itself can't cause it once key is off. You have no spark so it doesn't matter when spark occurs. Our old cars you can cut the key off even at off-idle, or key off and open throttle and it won't continue to run. It has to have something igniting fuel mix. Sent from my SM-S320VL using Tapatalk
Spark is a controlled heat source used to ignite the air/fuel mixture. If the timing is off far enough, it can cause enough heat in the engine (i.e. combustion chamber) to ignite the air fuel mixture without the spark. And every time you get a power stroke, each cylinder in the firing order is 1/2 revolution closer to firing. Basic answer is you have an uncontrolled excess heat source
Some of the late 60's early 70's hi-po Fords used a idle stop solenoid to completely close the carb butterflies to prevent run-on. Otherwise with high idle, deposits and so on the engine would still pull fuel and keep dieseling away.
Many years ago I worked at a local appliance store and one Dodge truck they had would diesel. Guys would shut it off, come in for lunch, go back and just turn key on and go, the whole time it would be chugging away. Sent from my SM-S320VL using Tapatalk
I’ve seen that before too, well almost that bad... and couldn’t believe the engine/pistons could take such a beating! Then I run a tiny bit too much timing......busted piston skirts! Bones
Yes that has to be really rough on the motor. Another place I worked for in mid 70s had 69 Chevy van, 250 cu. in. six. Guys always had me drive it because I made it backfire. Shut it off, pump throttle, back on...wham!!!!! It was funny for a good while until it threw a rod through the oil pan! Sent from my SM-S320VL using Tapatalk
So why are you arguing a point if you need verification and validation and assurance that you’re correct?
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