MG Car Repair: Shut off problem, oil pressure switch, charcoal canister

Question
I have a 76 midget when it warms up and you turn it off it just diesels for a long time after. What could the trouble be.Thanks

Answer
Hi Randy,
The 76 Midget like most of the emmission control cars has leaned out the mixture and over heated the combustion chamber in attempts to clean up exhaust emmissions. This produces the dieseling you are experiencing. This is "preignition/detonation" which ignites the fuel even without a spark. BLM stopped this run-on with an "anti-dieseling" valve. This valve opens a vacuum port to the carburetor float chamber so no fuel can leave the jet, thus stopping the dieseling or run-on after the ignition is turned off.

This system is not operating on your car. Power to the valve (next to the charcoal canister) is a Slate colored wire from the ign sw to a in-line fuse and on to the valve by a slate w/purple wire. The valve is grounded by a oil pressure switch via a slate w/yellow wire.

Howard