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