Dieseling

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Robert 411
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Dieseling

Unread post by Robert 411 » Sat Oct 07, 2017 2:17 pm

My 66 corvair 140 is dieseling. It's starting good, timing is good, air fuel mixture is good, and carbs have been rebuilt. It had been sitting for a while when i bought it. Could it not getting enough air flow through the engine?

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terribleted
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Re: Dieseling

Unread post by terribleted » Sat Oct 07, 2017 3:01 pm

Dieseling is generally caused by Idle speed too high, timing to far advanced, engine is too hot on shutdown, carbon build-up in cylinders causing hot spots that ignite the fuel. Over rich is also possible, but not getting enough air through the engine (causing a rich condition) is unlikely. If you are talking about lack of cooling air (debris blocking top of oil cooler or top of engine, thermostat doors not open once engine is warm etc.) causing overheating is certainly possible and could cause run on. Just because carbs have been rebuilt has little bearing on whether a carb issue exists.
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bbodie52
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Re: Dieseling

Unread post by bbodie52 » Sat Oct 07, 2017 3:48 pm

:think:
Wikipedia: Dieseling wrote:Potential causes

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.
  • Another potential "run on" problem after an ignition has been turned off can be attributed to an inlet valve(s) that are not sealing fully allowing the piston motion to pull a fuel mixture in when technically the valve is shut.
:link: https://en.wikipedia.org/wiki/Dieseling

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Here are some thoughts that I have of areas that I would check…
  1. The primary carburetors are sometimes discovered to not be contributing equally to engine idle. A number of instances have been identified where one primary carburetor is essentially "dead", with the idle circuits blocked or plugged. This leaves the engine idling on only three cylinders, with a single carburetor contributing all of the necessary fuel/air mixture to keep the engine idling. To accomplish this, it is often found that the one functioning carburetor has been adjusted improperly to hold the throttle open excessively at idle to keep the engine running, while the other carburetor contributes only air at idle. The faulty carburetor may actually "kick in" at speed, if the high-speed fuel circuits are not blocked. Since your 140 hp Corvair is fitted with a dual exhaust, you may be able to detect the possibility of this condition existing on your engine with a difference in sound or heat when comparing the left and right exhaust output. If this proves to be the case, the single carburetor that is contributing a fuel/air mixture coupled with a throttle butterfly that is held open excessively by an improperly adjusted idle speed screw might be the cause of the dieseling effect in your engine. If this were the case, the corrective action would be to dismantle, clean and overhaul the two primary carburetors to ensure that they are both equally matched, and then to carefully adjust and synchronize the two carburetors at idle to ensure that both are contributing equally to engine idle and that neither throttle is held open excessively. The throttle linkage should be completely examined and adjusted to ensure that none of the linkage is binding or preventing the two primary carburetors from fully returning to the closed idle setting.
  2. There is sometimes a possibility that one or both secondary carburetors is not fully closing when the progressive throttle linkage has released them to a standby position. The only spring in each secondary carburetor that forces the throttle butterfly valve to close is the internal spring that is a part of the accelerator pump in each secondary carburetor. The accelerator pumps apply spring pressure through an external link between it and the throttle butterfly linkage. These accelerator pump springs may not be sufficient to fully close a dirty or binding throttle assembly. A dirt buildup around the butterfly valves in the two secondary carburetors could also contribute to a problem that potentially would hold the throttle butterfly valves in the secondary carburetors partially open when they should be fully closed. This can cause a lean mixture condition by permitting excessive air to enter one or both intake manifolds. Throttles that are stuck partially open could also contribute to your dieseling problem.
  3. You should carefully check all of the vacuum connections described in the illustration to ensure a tight seal at each vacuum hose. The large vacuum hoses at the base of each primary carburetor can become distorted, brittle, or cracked and may not be sealing properly. All vacuum hoses and connections, including the PCV system connections and the vacuum break diaphragms that are part of each automatic choke on each primary carburetor should be checked to ensure that they are not causing a vacuum leak and a lean operating condition.
  4. Of course, ignition timing should be carefully checked and both the centrifugal and vacuum advance mechanisms should be checked to ensure that they are not stuck or holding the ignition timing in an advanced condition when they should have returned to neutral.
  5. If all of the above have been ruled out as a cause of engine dieseling, the possibility of carbon buildup and hotspots in one or more cylinders that could be serving to ignite a fuel/air mixture even when the ignition system and spark plugs have been shut down must be considered.
Brad Bodie
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Re: Dieseling

Unread post by notched » Sun Oct 08, 2017 8:35 am

Without going through dramatics. I have found the 2 leading causes are idle speed too high OR idle mixture is too rich.
Try lowering the idle speed and/or leaning the mixture out and see how it responds.
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