Moss Motors, Ltd

help me stop it please

There are any number of things that will cause engine run on, but one of the more common things is ignition timing too far retarded.  In an effort to stop pinging when running on lower octane fuel, the ignition is retarded.  This results in a lower idle speed, so the idle screw is run in to get the speed back up to a reasonable level, which opens the throttle plate and results in uncontrolled run on (because essentially the throttle is open when the ignition is turned off.  The fix is to bite the bullet and use a higher octane fuel, advance the timing and back off the idle screw.  Also, let the car idle for 15 seconds to a full minute before shutting the ignition off.

 

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Cheers,
Dave
http://homepages.donobi.net/sufuelpumps/

ddubois is right. The reason it keeps running is because all of the fuel is not being burned when it is supposed to be. The combination of the residual fuel, hot plugs and compression keep the engine dieseling after you shut it off.

Timing is usually the culprit, but dwell angle, spark plugs, fuel quality, compression, air intake and mixture all also play a part.  

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On their death bed, nobody ever said, "Gee I wish I'd spent more time at work!"

'68 Sprite 1275
'76 Midget 1500

Early carbureted engines (MGA and early MGB) continue fuel feed after ignition is switched off.  Cause of run-on is hot spots in the combustion chamber.  This hot chamber condition can be aggrevated by ignition being either a little early or a lot late.  But even with timing spot on some engines will run on anyway.  Any time the cylinder head is off you should take the opportunuty with a hand grinder to remove all sharp corners in the combustion chambers.

Later production MGB (with fuel vapor absorption cannisters) have an anti-run-on valve.  This device will draw a vacuum on the carburetor float chamber to stop any fuel flow to the main jet when ignition is switched off.  It works so well thet it will kill the engine immediately even if the ignition spark was still present.  It is not a particularly easy retrofit for the MGA.

My expedient solution to this problem is, step on brake, clutch down, select 3rd gear, and let the clutch up as you switch off.  It works perfectly every time, and no more run-on.

 

All good suggestions, though I'm not partial to the drivetrain shock from the stall-out method.

The first thing to do is adjust the idle speed to as low as you can get it, consistent with reliable and smooth running. Did someone mention letting the engine idle for a bit before switching off? This allows the combustion chamber(s) to cool a bit and lose the hot spots.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon