The MGA With An Attitude
Certain to prevent engine run on.

At 11:03 AM 8/27/05 -0400, Trackside wrote:
"After a fresh rebuild, and a light head deburing/polishing, if the problem continues (with proper tuning) what is a proper solution for an MGA owner (without bastardizing the wiring and having aftermarket gizmos hanging in the engine bay?"

No special equipment necessary. Depress brake, depress clutch, slip into 3rd gear, switch off engine, release clutch as engine is coming to a stop. Works every time, no run on. It may be easier to develop this habit than to modify the car.

"Could a correct MGB set up be fitted to the A with a B ignition switch or would our positive ground set up interfere?"

Yes, the B parts could be used on the A, although you do not need all of the B emissions parts to do it, and power polarity does not matter. Out of habit I will call the B anti-run-on valve the "ARO valve", although the book commonly calls it run-on-control valve (ROC ?)
MGB type ARO valve
The MGB standard ARO valve (Moss 367-110) is a solenoid operated low pressure 3-way air valve. This normally vents air from the absorption canister (side hose) to atmosphere (bottom hose) at all times except when actuated for the ARO mode momentarily after ignition switch off. It also has a vacuum supply hose which is connected to the intake manifold. During ARO activation the valve closes the atmospheric vent to induce vacuum in the absorption canister. Vacuum in the canister then acts through the vent lines on the carburetor float chamber(s) to draw the vacuum on the float chamber, thereby stopping any fuel flow in the carburetor main jet(s). No fuel, no flame, no possible run-on, regardless of hot spots or bad tuning.

The 1973 and later B ignition switch is designed for steering column mounting, not easily adapted to dash mounting. So it would likely be easier to use a double throw relay to give the normally closed contact to be hot with ignition switch off. This simple small relay could be hidden behind the dash near the ignition switch. Or it might be mounted near the fuse block, or anywhere else where it is convenient to hook up to existing white, brown (or brown/blue), and black wires. White and black trigger the relay when ignition switch is on. Brown supplies power through the NC contact to operate the ARO valve. Brown wire with stripe (other than blue or green or black) is appropriate for switched hot wire, or Slate/Purple as in the B.

You could use the B oil pressure switch (Moss 141-715), mounted in a T fitting in the MGA oil pressure gauge signal line. You might use the B "T" fitting (Moss 180-245) in place of the original oil gauge connector fitting, hose to pipe (Moss 435-530). The pressure switch might be hidden behind the dash if you were crafty enough to install a "T" on the back of the oil pressure gauge. The pressure switch runs to ground on the chassis and has one wire to connect to the ARO valve. Black/white wire is appropriate for a switched ground wire, or Slate/Yellow as in the B. As such, the power supply relay and the pressure grounding switch each need one wire run to the ARO valve.

You can use this ARO valve on the MGA with minimal plumbing. First connect hoses from the carburetor float chamber overflow pipes to a T fitting, and connect that to the "canister" port on the ARO valve. The "vent" port on the ARO valve should be drained (large vent hose on bottom) somewhere clear of the hot exhaust parts, in case of carburetor fuel overflow. The third port on the ARO valve needs to be connected to intake manifold vacuum.

If you wish to "hide" the ARO valve (best as possible) you might mount it on the side of the engine aft of the left engine mount near the original carb vent pipes. In the case of the Twin Cam car, ditto but on the right side of the engine. You might otherwise mount the ARO valve on the heater shelf aft of the intake manifold. This also requires the vacuum source hose (metal pipe recommended near hot exhaust components) to run from the ARO valve to the intake manifold, and two connector wires for the ARO valve (previously described).

In this configuration the float chamber(s) vent freely through the ARO valve to the drain at all times, except when the valve is actuated during ARO mode. When you switch ignition off the ARO valve will be activated, closing the drain port and connecting vacuum to the float chambers. Once the engine stops turning the oil pressure switch will break the ground connection to deactivate the ARO valve. Whenever ignition switch is on the small relay disconnects signal power for the ARO valve.

Shopping list is:
ARO valve
Oil pressure switch
Oil pressure T fitting
Small relay with NC contact
Miscellaneous hoses and wires.

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