Stir up a little trouble of run on

David G

Well-known Member
My interpretation of run on is that the engine continues to run after the ignition is shut off due to spontaneous combustion in the cylinder.

Causes

Loose Carbon in cylinder
Excessive heat in cylinder combined with low octane fuel.
throttle plate open to far due to idle stop set to high.



Timing to advanced has NO effect
Timing to retarded only has an effect of causing excessive heat in cylinder
Ignition circuit has no effect

Discussions welcome.
 
Try to lean the carb. A lean burn is cooler. Get is as lean as possible without engine stumbling when put under load.

May try blending a little E85 with gas, say making it E15 to start with. That should make it leaner too.
 
For purposes of vintage tractors, the overwhelming cause of dieseling run-on is idle speed screw holding throttle plate open too far.

Retarded ignition timing can contribute due to increased cylinder temps.

Fuel octane not an issue.

Spark ignition circuit not an issue by definition.

FWIW: Spark ignition engines can run on in reverse direction for several seconds. In such situations, the engine oil pump is also running in reverse direction.

Dean
 
Hey there George, with all due respect Id like to take an opposite conclusion then yourself but NO WARRANTY you may well be right, I'm an Electrical NOT Mechanical Engineer remember.

You state " Try to lean the carb. A lean burn is cooler"

Its my opinion that gas (flow in and out of the combustion chamber) is a better heat conductor and transfer agent then air, so if the fuel air mixture has more gas then air (richer) the engine would run COOLER. Air just doesn't absorb hold and transfer heat as well as a liquid in my opinion so it looks to me like if the fuel/air mixture is richer it will carry more heat out of the combustion chamber then if it were only air or a leaner mixture.

We may have to arm wrestle lol or agree to disagree and remain friends of course.

Any fuel experts or mechanical types out there who know who is correct???

Take care George

John T
 
George, I take the opposite opinion. Lean is not cooler. The fuel flowing thru the system has a cooling effect. When flying an airplane. on takeoff the throttle is kept in full throttle and full mixture to keep fuel flowing through and cooling the engine. Its only until your are safely aloft that you reduce the throttle and lean the mixture. I take that to apply to all gasoline engines.
 
If I had a rig that tended to diesel at shut off, I was always told to have it in gear when shutting down, then release the clutch (with brake applied) immediately to end the dieseling as quickly as possible. Had never thought about the oil pump running the wrong way- that would sure be a good reason to bring it to a screeching halt.
 
As we all know, if you get gasoline on your hands it feels cool as it evaporates...that said...we have all got alcohol on our hands and that cools also..due to the fact that alcohol (methanol or ethanol) has quite a few BTU's less per pound, anytime an alcohol is used for fuel, it has to be introduced into an engine quite a bit richer than gasoline to keep the engine running properly. Because of the huge amounts of alcohol, it cools the engine down extremely well, so well that some of the super mods running at the local tracks have an extremely small radiator...usually a little bigger than a heater core. Those engine make bookoo horsepower and stay cool doing it..if an engine runs lean..it will get hot in a hurry and will burn holes through pistons........
 

Google engine runs after shut off:

Dieseling can occur for several reasons:

Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for igniting unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, rough metal regions within the piston chamber can cause this same problem, since they can glow red. It has also been suggested that an improperly rated spark plug can retain heat and cause the same problem.
A carburetor that does not completely close can contribute to running once the engine is off, since the extra fuel and oxygen mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcase can provide ample fuel for dieseling.
Incorrect timing.
An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to burn.
An idle speed that is too fast can leave the engine with too much angular momentum upon shutdown, raising the chances that the engine can turn over and burn more fuel and lock itself into a cycle of continuous running.
 
Not a fair analogy. Supermods only run for a short time, a quarter or eighth of a mile, and are then shut off. Also note that they get overhauled or scrapped after very few runs. If you tried to run a tractor that way, you sure wouldn't get much done.
 
OK, let's flog this one to death....
Of what I know about engines.....
1) It is a proven fact that leaner mixtures burn hotter. That is why modern engines use EGR - to COOL and DILUTE the combustion to reduce peak temperatures. This results in less formation of NOx.
2) Retarded timing will also make an engine run hotter. Just look in any old auto repair manual (like an OLD Motor's or Chilton'e). One of the listed leading causes of over heating is retarded timing followed by mixture too lean.
3) Fuel is introduced into the combustion chamber as a VAPOR, and should therefore have minimal cooling effect.
3) OCTANE! Most people do not know what octane is or what its effect is. so....
a) Octane is a comparison to what was once designated as the "ideal" motor fuel....Isooctane.
b) Octane is determined by flash point and rate of burn when ignited as well as Reid vapor pressure. Lower octanes have lower flash points and faster burn. In view of this, I would say that octane most certainly DOES have an effect on run-on!

The bottom line is that you still need fuel mixture, compression, and an ignition source for an engine to run or run on. Given that the compression will be present after the engine is turned off, and that fuel mixture will also still be available, the source of ignition comes into question.
Given that after the switch is turned off, the engine parts are still moving, and are still hot, I would say that my conclusions are as follows:

1) Reducing the idle setting would also reduce the likelihood of running on.
2) Higher octane fuel would also reduce running on due to the higher flash point.
3) Timing is irrelevant.
4) Reducing operating temperature should also have an effect on running on.

OK. I said my piece. Let the beatings begin......
 
George,

I believe google is saying that a combustion chamber will carbon up when to rich, the unburnt carbon can flake off and cause dieseling.

I do believe a mixture that is too lean will run hotter, because the heat is carried out by the excess hydrocarbons.
 
David G,
When younger a friend had a small airplane. At higher altitudes, the air is less dense and engine would run rich. To adjust the carb, he pulled on a lever which made the mix go lean. He used an exhaust temp gauge to get the mix perfect. As you lean the carb out, the exhaust temp will increase to a point. After you get to max temp and you continue to lean carb, the exhaust will decrease.

In the early 70 Chrystler experimented with a lean burn engine, before fuel injuection and the modern computers. So going too lean will actually cool engine. Chrysler also played with varing the timing to compensate for engine pinning and engine temps.

After I couldn't get leaded 98 octane for my 62 XKE, I had to retard the spark.

I recently purchased an exhaust temp gauge, $22 off ebay and installed it on my Jubilee. I cranked the main jet in 2 1/2 turns from winter setting to get max exhaust temp. Another 1/8 turn in and exhaust temp decreased big time, just like the airplane. My spark plugs look like new, no carbon deposits.

Other options: My IH C has notches on the throttle settings. When I bought it, someone set the carb so in the lowest position the butter fly in the carb was completely closed and engine would die.
I have to put throttle on the 3 notch for it to idle properly. It's like turning off a diesel. Restrict air flow and engine stops.

My Terramite has a gas solenoid valve. To stop the 20 hp kohler command engine, the ignition switch, in the off position, would short out both coils, just like a lawn mower. I discovered that the engine would die in about 5 seconds if I removed power to solenoid. So I removed the coil wire from switch, turn key off and let the engine run out of gas in 5 seconds, no dieseling.

Recently I installed a gas solenoid on my Jubilee because the carb leaked gas on barn floor. If my Jubilee decides to diesel on a hot day, I will do the same thing, remove power from solenoid and let it run out of gas.

Try simply turning the gas off and let your tractor run out of gas. Bet it can't diesel with no gas in carb. Please try it and post back.

Yes, in my opinion, carbon will cause dieseling. Carbon is caused by too rich of a mix. Lean out your carb. A rich mix will also cause the oil on cyclinder walls to burn off, causing excessive ring wear.
George
 
"I do believe a mixture that is too lean will run hotter...."

Agreed, David.

This is well settled.

Dean
 
JohnT,
Yes leaning out a carb causes an increase in temp up to about 15:1. Going past that point, the exhaust temp will decrease. I just put a temp gauge on my exhaust and temp will decrease so will horsepower. So find a point past 15:1 and it will run cooler. Going to far, when governer opens engine will stumble. Find a happy mix stop making carbon deposits.

In my case, in the winter, I need to richen up carb 2 1/2 turns. Cold winter air and winter blend gas.

Same is true with old carb airplanes. I've seen done on an airplane and I've done it with my Jubilee.

Too rich mix, causes carbon build up in combustion chamber, causes increase in compression ratio, hot carbon acts like a glow plug.


George
 
The worst case of run on I experienced was in the early 70s when I worked for a canning company during the summer. The two way radios we used to communicate where tube type and would drain the battery in about an hour. As a result the engine in the 64 GMC pickup that I drove idled all the time. If I shut it off after a long time of idling I could put the truck in gear and drive off with it it would even accelerate a bit if you gave it a little throttle.
 

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