Run-on by dieseling is caused by fuel in engine igniting without spark.
Possible explanations:
a.) Running winter fuel in warm weather.
Winter formulated fuels have a higher vapor pressure, meaning that they evaporate easier. Only vaporized fuel will burn, not the liquid stuff. The cure is to run the fuel tank nearly empty and fill up fresh. The oil companies change the fuel formulation seasonally, and sometimes even monthly.
b.) Using gasoline with alcohol content.
Gasahol gasoline contains about 10% alcohol, usually methanol alcohol. The alcohol vaporizes at a much lower temperature than the rest of the fuel. The result is alcohol vapor in the combustion chamber, and the engine runs on easily. This stuff can also cause vapor lock in a carbureted engine on a hot day, or when you shut off a hot engine and then try to start it five minutes later, after heat soaking the carbs under the hood. This is a nasty problem with my MGA, with only a feed line to the carbs -- no return line. A carb with a fuel return line to the fuel tank will help ease this problem. Keeping the fuel circulating back to the tank can keep the fuel and the carb cool. Also carbs located on the same side of the engine with the exhaust manifold are especially susceptible to this problem. My solution is to stay completely away from the alcohol content fuels.
c.) Using low octane gasoline in an engine that should have high octane fuel.
High octane fuel is more resistant to both evaporation and pre-ignition. Check your owners manual (if you still have one) for the recommended octane requirement. For good measure, add one or two points to the recommended number. The method of rating fuel for octane level changed some years ago, and the numbers are now a couple of points higher to achieve the same results.
d.) Run-on valve defective or out of adjustment.
This function does not apply to an MGA, but is included here as a guide to other models. The function of this part is to completely shut off all fuel to the engine when you turn off the ignition. On a carbureted engine you may otherwise find a run-on solenoid. The function of this part is to completely shut off the air intake at the carb when you turn off the ignition. If either of these devices is out of whack, fuel/air mixture can get into the engine after shut-down, causing run-on.
e.) Hot spots in the combustion chamber.
Any carbureted engine having neither of the above mentioned devices will continue to draw in fuel/air after shut-down. Any hot spot inside the combustion chamber can ignite the fuel causing run-on. Right off hand I can think of at least three sources of hot spots.
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