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Engine Run-On
I have an ERA 723 with an MSD box. The engine idles best between 900-950. When the engine is shut down it will try to run on. If idle is slowed to 700-750 it no longer runs-on.The MSD instructions mention this and suggest a diode in the line that goes to the charging indicator. Question is this the G/R wire that goes to pin 1 on the voltage regulator?
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I tried the diode trick and it didn't work. Mine idles at 1050 rpm. 'm just use to putting it in second and letting the clutch out when turning off the key.:o
Scott |
It's not the MSD, it's your high idle speed. The higher idle speed means you adjust your carb butterflies open "just enough" to sustain the 900 to 1000 ( or more in some cases) idle rpm. Those carb butterflies are open "just enough" to allow the engine to continue to draw air in through the carb, mixing gasoline with that air, and the fuel/air mixture is then ignited by the hot combustion chamber, instead of the spark plug. Because this is now a "random firing event" based on the heat of compression and the hot combustion chamber the firing is not "timed". Thus the motor runs terrible, it runs, but not well. It may even at some point briefly run BACKWARD. Which is just hell on the timing chain and other internal components, very nasty this dieseling effect.
Kill it with the clutch at the same moment you turn off the key. Or reduce the idle speed by closing the carb butterflies "just enough" to prevent run on. Now if it is running really smooth, as if you never turned off the key at all but you really did, THEN you might need to address the MSD thing. |
You are both right and it shuts down by letting the clutch out. Thought I would try the diode. Thanks for the help.
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My Kirkham is doing the same thing and Tom recommended the clutch trick and it worked like a charm. I really appreciate the deep explanation Excaliber -
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I had the same problem with my 408W since it likes to idle at 950+!!:cool:
So I fabricated a bracket to hold a fast idle solenoid that holds the throttle slightly open (950 RPM) when energized and allows the throttle to completely closes when the ignition is turned off.....problem solved!!:D If interested it is an A/C throttle position solenoid used in older cars.....simple and inexpensive!!;) |
Both of my cars idled at 1100 RPM and I never had any problem with them running after I shut the key off. And sometimes they were pretty hot. Wonder why I never had a problem, not that I am complaining. The electric Solenoid is a great thing as I have seen others use them.
Ron |
I've experienced run-on several times over past years.
I usually have the initial timing set at about 15-16 degrees. Anything higher than that on my engine has contributed to run-on. When run-on set in, I dialed the timing back several degrees and was able to eliminate the problem. David |
The higher the octane the gas is the less likely run on will occur. High octane fuel is actually harder to ignite than low octane is. That is the fundamental difference between the high and low, one is more stable than the other.
Running race gas, extreme octane? Run on won't be a problem. David that an interesting observation about the timing effect. Of course once you turn off the key it doesn't matter where the timing used to be, it aint there anymore at all. So, what could be happening with reduced timing not promoting run on? Possibly a cleaner burning combustion chamber or a slightly cooler combustion chamber by "just enough". |
Coosawjack, I like that idea and may give it a try.
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Myself, I use the vacuum advance on my distributor. The vacuum advance increases my idle rpm WITHOUT having to open the carb butterflies to increase the idle rpm. When the motor is turned off and the electric timing (distributor) is no longer in play the carb butterflies are all ready closed off enough to prevent more fuel/air from being drawn in. Similiar to the solenoid approach, closing off the carb butterflies in some manner. In the early days of pollution control which required a higher idle speed the solenoid method was a common tactic used by many manufactuers specifically to address the run on issue.
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Back to the origninal question about the diode. The diode goes on the wire that connects to the I terminal before your volatge regulator. If you have a charging light it goes between the light and the I terminal. This prevents current flowing backwards from the alternator to your ignition system after the ignition is shut off. This in combination with to high an idle and or to advanced of timing contributes to run on or dieseling. Like has been said, the solenoid works good, but I have seen some cars with a solenoid continue to run without the diode. This can be caused by hot desposits in the combustion chamber and with possibly the regular idle screw not backed out far enough. A leaky power value also can contribute to the problem. I believe you can buy the diode from Haywire.
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You know if the motor was still running because the MSD is still getting electric power then it would seem the motor would run/idle fine. Nice and smooth, timed correctly, electric spark from the spark plug igniting the fuel/air mixture at the proper time. It just won't shut off, yeah you could call that "run on", but the term may be incorrectly used in that manner.
"Run on" or "dieseling" most generally refers to a condition where the spark plug is NOT firing, the motor is STILL running but very rough, likely it is backfiring through the carb, it may even be running backward at some point just before it dies and in general sounds terrible! The MSD problem is troubling and a hassle but it won't damage your motor, whereas "run on" or "dieseling" could seriously damage your motor. So, is this "run on" condition of which we speak "smooth" or "rough" in nature? To entirely different conditions. IF the MSD does shut down within a few seconds after you turn off the key you might THEN have a "run on" condition that is seriously rough in nature. Fixing the MSD won't make the second part go away. |
I'd try higher octane fuel.
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Actually, the current that you get that runs back to the ignition from the alternator without a diode is a very weak current, which results in the very rough, just barely still turning condition. Kind of like an engine running with a weak coil, or like a wet distributor cap. I agree run on or dieseling can damage engine parts and is the primary reason the condition needs to be fixed.
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