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MG MGB Technical - can the anti run-on valve prevent starting?

My '73 BGT was running great until this week, when it decided it would not start - carbs were bone dry. I suspected the fuel pump. Replaced it and got started. Next morning, the new pump would not start the car either. Is is possible for the anti run-on valve to malfunction when the ignition is turned off, and then prevent starting?
Patrick Callan

The anti-runon valve will only activate when the ignition switch is in the off position. The fact that you don't get any fuel with the OEM SU fuel pump and the aftermarket pump indicates that you are either not getting power to the pump or there is a clogged inlet to the pump. See the article, Fuel Delivery Troubleshooting Guide in the SU Fuel Pump Articles section of my Homepage at: Cheers - Dave
David DuBois

Thanks for the link. Will spend some time studying the guide.
Power to the pump is fine, and once I got the car started, it ran great. Only when I left it over night did the problem repeat. Battery has been tested, and is fine.
Is it possible for the run-on valve (or maybe the oil pressure switch) to keep the valve activated when the ignition switch goes to the on position. Not sure if there is an intermittent problem - it started all day yesterday.
Thanks again for the link - all great info.

Patrick Callan

Yes the anti-runon system can prevent starting - if it is working incorrectly. However it would be an unfortunate coincidence if this happened the day after changing a faulty fuel pump. If left for several hours does the new pump click a few times when turning on the ignition? It should.

A problem with the oil pressure switch will prevent the valve operating when it should, it won't cause the valve to operate when it shouldn't.

The anti-runon valve operates when there is oil pressure closing the oil pressure switch which sends an earth to the valve, and there is 12v coming from the ignition switch in either of the 'off' positions, i.e. including when it is in the accessories position. Normally that only happens when the ignition is turned off with a running engine of course. But if the ignition switch fails to disconnect 12v from its wire when the ignition switch is turned on, the valve will operate when it shouldn't. However I'd expect that to allow the engine to start but cut out again as soon as any oil pressure is developed. If this is happening disconnect a wire from the valve and see if that stops it.

The problem will also happen if the valve is stuck in the operated position even though it is unpowered, or if anything blocks the large port at the bottom of the valve. Possible on late models with the dual canisters, the second of which is connected to this large port, less so if the port is simply vented to air as on later models. In this case the engine probably won't start at all, disconnect the hose between the canister and the carb float chambers to check for this.

A large vacuum leak will also stop an engine starting, as well as lack of fuel or lack of sparks. Normally those two things would be the first thing to check for.
Paul Hunt

I'm beginning to wonder if the original fuel pump was at fault, because it (the pump) was running when the ignition switch was on - and the same symptom happened with the new fuel pump. In both cases, I was able to get the car started (eventually), but the next day it was back to no-start. I have it in the shop right now for some new steering rack boots, and they said it started fine all day yesterday, so we will see what happens when I get the car back.
I forgot about the large port on the valve. Will also check that, and if it happens again, I will try disconnecting the valve and/or the switch to see if there is any impact.
Thanks for the info,
Patrick Callan

Pat - See the articles, Fuel Delivery Troubleshooting Guide and Air Leak Troubleshooting in the SU Fuel Pump Articles section of my Homepage at: Cheers - Dave
David DuBois

If the pump was chattering away but failing to fill the float chambers then you could have a blockage in the pipe from the tank to the pump, an air-leak in the plumbing on that side, or indeed a pin-hole in the pickup pipe. The latter will often go away if you fill the tank.

If the pump failed to click it may well be the power supply to the pump that is at fault. This consists of the white wire from the main harness to the rear harness in the mass of bullet connectors by the fusebox, and an earth from the rear number plate bolt, as well as the connections at the pump itself.
Paul Hunt

Well, picked the car up from having new tie rod ends installed. The service station said it started every time for them, and started nicely for me on the trip home. We'll see if the symptom returns.
Maybe a temporary fuel line blockage, maybe an intermittent problem with the oil pressure switch and/or run-on valve, maybe ...

Thanks for all suggestions and links. Will advise if it happens again.

Patrick Callan

This thread was discussed between 08/05/2013 and 18/05/2013

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