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