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.

1.) Spark plugs of the wrong temperature range can run red hot at the tip. This can also lead to premature failure of the ceramic insulator near the tip of the plug. Check the service books for the correct spark plug number.

2.) Carbon build-up in the combustion chamber. The carbon deposits have a rough and irregular surface with many small cracks and edges exposed at the surface. Carbon deposits are also a poor heat conductor. The combined effect is extremely hot bits of carbon which can ignite the fuel after shut-down. For especially heavy deposits, the only solution is removal of the cylinder head and physically scraping away the deposits.

In milder cases, I have known of limited success with the use of "mouse oil". This is a generic term applied to anything liquid that costs two cents to bottle and sells for more than a dollar. (STP Gas Treatment and Marvel Mystery Oil may fall into this category). The primary ingredients are often nothing more than kerosene, lacquer thinner, and the like. It is often called "upper cylinder cleaner". Instructions for use may call for pouring the stuff down the carburetor at fast idle with a hot engine, pouring in the entire container full at a slow trickle over a period of five minutes or so. This usually causes a lot of smoking from the rich mixture, and sometimes pinging from pre-ignition. While the pinging is OK for this short procedure, don't pour the stuff in so fast as to cause detonation (severely loud pinging), as this can damage the engine.

You may also add this stuff to your gas tank. The results here are not so obvious and definitely not immediate, but in long haul it MIGHT clean you engine inside (no guarantee) or may be intended to help keep it clean. Personally I think this type of application is more effective a cleaning the parts in contact with liquid fuel, such as fuel jets in a carburetor and fuel injector nozzles. Many of these compounds are already included in certain brands of detergent gasoline, so you may be wasting your money here by duplicating the additive.

3.) Sharp edges on metal parts in the combustion chamber. These sharp corners can get red hot while the engine is running because of poor heat dissipation. The hot edges can ignite the fuel after shut-down. These sharp edges also have a few sources.

On an older engine, when valves are severely burned they can develop cracks or sharp edges. If an engine shop does a "cost sensitive" valve job, they may reinstall valves that have been ground too far, exposing sharp edges on the outside of the valve heads. These sharp edges on valves are a definite no-no! Do not attempt this false economy. Replace any valve with a thin edge.

When doing a valve job, it is common to resurface the head (milling or surface grinding) to assure a flat surface for the head gasket to seal against. This will definitely leave sharp edges at all vertical surfaces. You want to remove these sharp corners with a hand grinder, producing a small bevel or preferably a rounded corner.

Many cylinder heads come from the factory with built-in sharp edges in the form of "casting flash", especially older technology parts. The cure for this is also removal of the cylinder head and physical removal of the rough bits with a hand grinder. The original cylinder head may also have sharp corners within the combustion chambers. You especially want to check the apex of a heart shaped chamber at the center of the heart shape between the intake and exhaust valves. This is very near the center of the combustion chamber and subject to the highest combustion temperatures. Put an especially generous radius here.

Reboring, ridge reaming, and/or honing of cylinders can also leave a sharp corner at the top of the cylinder bore. Resurfacing of the engine block will produce the same sharp corner. Ditto removal of the sharp edge with a hand grinder.

    f.) Spark timing adjusted outside of specified limits.

Spark timing advanced too far can cause pinging and detonation, both indications of firing too far before top dead center. Here the combustion pressure is trying to push the piston down before it reaches the top of the stroke, trying to make the engine run backwards, fighting the forward motion, wasting energy (and fuel), and generating excess heat from the wasted energy.

Spark timing retarded to far will cause late combustion, losing the opportunity to take advantage of the most efficient part of the power stroke, at the top where the highest pressures occur. If the timing is just a little late the result is a slight loss of power and a little dip in fuel economy. But the fuel IS being burned, and the heat is going somewhere. If it doesn't go into propulsion, it goes off as excess heat. The extra heat may go into the cooling system (high water temperature), or may cause hot spots in the combustion chamber. In severely retarded timing cases and at high engine speeds, the last of the combustion may occur as the gases are exiting the engine, causing overheating of the exhaust valves, exhaust manifold, catalytic converter (not MGA), etc, all very bad news.

    g.) Fuel mixture adjusted outside of specified limits.

A lean mixture leaves excess oxygen after combustion. Besides possibly causing you engine to ping and to run hot and to run on after shut down, the left over oxygen can combine pyro-chemically with the aluminum in the pistons, burning holes through them in places where you'd rather it didn't. If you're heavy into high compression or off road racing, you may want to adjust the mixture somewhere between slightly rich and flooded to protect the pistons.

A rich mixture leaves soot and excess fuel after combustion. The soot will leave carbon deposits in the combustion chamber and on the spark plugs and can foul the piston rings causing loss of power and/or excess oil consumption. It can also leave deposits between the valve head and the valve seat, especially the exhaust valves, causing loss of compression and burning of the valves and seats.

Excess fuel otherwise exits via the exhaust valves into the exhaust manifold and throughout the rest of the exhaust system. Many post-1967 engines have an air injection pump which puts fresh air into the exhaust ports or manifold to complete the combustion of left over fuel. While this does nothing for power output, it does reduce hydrocarbon emissions. In the process, burning the excess fuel in the exhaust system generates excess heat in all the wrong places. This can lead to premature failure of any or all of the exhaust components (including that expensive catalytic converter), and can cause fires in nasty places where they don't belong, like your carpeting, the oil deposited under your British car, or in the roadside grass if you park there.

If you systematically eliminate all of these causes, run-on should be banished forever. Not all of these things will always cause run-on. As a result, you may find the cause of your particular problem and eliminate that problem along with the run-on, and still have some of these other problems. The best approach is to start with a complete tune up. and then do a compression check.

Compression should be at least 120 psi, and all cylinders should have the same pressure within 10 or 15 psi. You may find pressure of 175 psi if you have a high compression engine. If you find three cylinders at 125 psi and one at 175 psi, suspect gross deposits in that one cylinder.

If you should encounter anything not on this list as being the cause, please let me know so I can add it to the data base.