The running-on control valve is fitted to MGBs' manufactured after 1973. It is located on the right (passenger) side of the engine compartment near the firewall, forward of the vapor cannister, and controlled by an oil pressure switch mounted on an oil line lying flat on the firewall shelf next to the vapor cannister. This valve is designed to transfer carburetor constant depression to the top of the float chamber at the point of engine shut-down, thereby preventing any fuel flow from the carburetor jet.

The unit comprises a solenoid operated two-way valve and is located in the vent line leading to the base of the evaporative loss canister. The carburetor constant depression area is connected by a line to the running-on control valve, and when the solenoid valve is energized by switching off the ignition, vacuum is momentarily routed to the carburetor float chamber via the evaporative loss canister.

Once the engine stops rotating, an oil pressure activated switch opens due to the diminishing oil pressure as the engine comes to rest, thus allowing the solenoid valve to open the vent line to atmosphere. Even if all other emission equipment is to be removed from the MGB, this valve is worth maintaining because it prevents potential damage to engine bearings and valves caused by running-on (dieseling) as the engine tries to run backwards. Running-on is caused by the modern lower octane fuels. Keep the valve. Its presence does not consume additional fuel or harm performance in any way, and it does offer worthwhile protection.

Check the operation of the running-on control valve with the ignition off and key removed. Remove the slide connector (slate/yellow wire) from the oil pressure switch. Ground the connector. You should hear the valve operate. If you do not, use a test light to see that the grey/purple wire connected to the top of the valve (terminal #1) is hot. If it is not, the problem may be in the running-on control valve line fuse (fuse with a grey wire on one side, grey/purple wire on the other). Trace back through the circuit to find the problem. Note: If the ignition is on, the line will not be hot.

If it is hot, reconnect it to the valve. Check to see that the other valve terminal (#2) has power. If it isn't hot, the valve (Moss #367-110) must be replaced. It is not repairable. If it has power, reconnect it and check the connector at the end of the wire where it plugs into the oil pressure switch. If the end connector has power but does not, when grounded, activate the valve, the running-on control valve is stuck and must be replaced.

Check the oil pressure switch (#141-715) with the engine off. To test the switch you must reverse the function of the test light. Usually a test light lead is grounded to search for hot terminal (the #7 fuse terminal in the fuse box is a good choice). Now when the test light comes on you know you have a ground.

Test the base of the switch. The light should glow, as it is grounded well. Now check the male connector on the oil pressure switch. The light should not glow as it should not be grounded. This terminal is grounded only when oil pressure is in the system, as it runs and shuts down. If it does glow, the switch is faulty.

A faulty switch need not always be replaced. Most of the time the switch can be cleaned. Oily vapors coat the inside of the switch after a period of time and it becomes sticky, slow or incapable of operating. Usually the switch sticks in the closed (male terminal grounded) position causing the running-on valve magnet to operate continually when its action should be only momentary.

Remove the valve to clean it. Squirt some carburetor cleaner in the bottom hole of the valve. Shake the switch and continue cleaning until it rattles. Re-install and check with a test light connected to a hot terminal: with the engine off the male switch terminal should be dead (neither hot nor grounded). With the engine on, it should be grounded (hot, as the test light is on a hot terminal).

PROBLEM
Wire to R-OC valve hot at terminal #1 (GP wire).
SOLUTION
Engine off-normal condition
Engine on-fault in ignition switch.

PROBLEM
Wire to R-OC valve dead at terminal #1 (GP wire).
SOLUTION
Engine on-normal condition
Engine off-blown line-fuse, bad wire, try cleaning line fuse contacts.

PROBLEM
Terminal #2 (GY wire) on valve dead while terminal #1 (GP wire) is hot and connected.
SOLUTION
(Engine off) clean terminal and connector make sure wire connected is SP and hot faulty valve, replace.

PROBLEM
Terminal #2 (GY wire) on valve hot, but connector at oil pressure switch dead.
SOLUTION
(engine off) clean terminal and connector replace wire and connector.

PROBLEM
Oil pressure switch reads grounded.
SOLUTION
Engine on-normal condition
Engine off-clean switch or replace it.

PROBLEM
Oil pressure switch reads dead (neither hot nor grounded).
SOLUTION
Engine off-normal condition

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