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MG MGB Technical - Run-on

My '73BGT has started running-on when the ignition is turned off. The anti-run-on switch seems to be "clicking" as described in my shop manual for the test, but it is evidently not functioning as required.
Are there any other related things that I should check?
My troubleshooting manual indicates that timing may be an issue.
Any help/ideas much appreciated.

Pat
Pat

Pat: What octane gas are you using? I use Sunoco 94 in my 69B (desmogged, and no anti-run on valve) and have never had this problem, even on very hot days.
Andrew Blackley

Could also be that your idle speed is a bit high. Or maybe you got a bad load of gas.
Mike MaGee

The North American anti-runon system is so effective it renders octane and idle-speed considerations irrelevant. I can say this with some confidence because when later models were provided with the ignition relay a wiring fault was introduced which meant that when the ignition key was turned to 'off' power was left connected to the coil so sparks continued to be generated. This gives rise to the 'normal running' type of runon as opposed to the Dieselling type of runon when there are problems with the anti-runon system. If the valve is clicking there is an air leak or blockage in the emissions plumbing. Carb vacuum is applied to the front tappet cover, there is a sealed oil filler cap, and a restricted port on the back of the rocker cover. This goes via a pipe to the top of the charcoal cannister. Other pipes go from here to the tank and the carb float chambers. The bottom of the canister goes to the valve, which has a large port open to the atmosphere, and another pipe to the inlet manifold. When you switch off the engine the large port on the valve is closed off, so vacuum from the carbs and inlet manifold is applied to the float chambers which sucks the fuel out of the jets stopping the engine from firing instantly - but due to lack of fuel not lack of sparks. If any of these pipes, or the tank or its filler cap or the oil filler cap let in enough air due to splits, coming loose, or incorrect items there will be insufficient vacuum applied to the float chambers. Alternatively if the float chamber pipe has become blocked the same will apply, but in that case you are likley to get very rich running with fuel pumping up out of the jets. You can see details of the emissions plumbing and anti-runon system at http://www.mgb-stuff.org.uk/wn_fuelframe.htm and click on 'North American Emissions Plumbing'.
Paul Hunt

Pat, Choke it before you shut down.
Dwight McClain

Thanks all,
I'll try to check all the "plumbing" this weekend. I have started using "regular" gas, though - so I may also try a fill up with higher octane. Not supposed to cause this, but it's an easy check (and it wasn't doing this before, when I was using 93 octane).
Will also check for splits or mis-connections in all the hoses and joints in the emissions stuff - have a feeling that is the problem.
I did notice that when I stop for gas, there is a major relief of pressure when I remove the gas cap - don't remember that before.
Again, thanks for all the comments and suggestions. I'll check all of them, and will try to follow-up.

Pat
Pat

With an emissions system, unlike those with a sealed tank and a vented cap funnily enough, there should be no release of vacuum when removing the cap. This sounds like another problem. If completely sealed due to a blockage this would normally cause petrol starvation after a few miles. The vented cap causes a release because it should have a spring-loaded valve, rather than just an open port, to prevent leakage in a roll-over.
Paul Hunt

Paul,
I guess there could be some sort of blockage, but I would have expected other symptoms. Aside from the run-on, idle is smooth, mileage seems to have improved (since I replaced two flooded floats), and there doesn't seem to be any fuel starvation.
The only other symptom that I can think of is a popping from a hole in the exhaust when coasting down hill - and that is not all the time (may be somewhat air-temperature-related).
So, I'm not sure the tank vacuum is present when the engine is running. Any way the run-on could create the vacuum on shut down?
Thanks for the added info,
Pat
Pat

Paul,
Another follow-up. I read the info on your website, and have a question - the instructions on how to test the anti-runon switch indicates that the ingnition should be on, but engine not started. The manual that I have does not mention the ignition switch, so I have been checking the switch in the same manner that you describe, but without turning the ignition to ON.
Should the switch be able to operate without the ignition ON? If no, I may have some sort of wiring issue. Don't know if that could be contributing to my problem, or not.
Thanks for any additional info,
Pat
Patrick Callan

Paul,
Would you expect this to work any differently with a Weber DGV downdraft carb. My anti-run on switch seems to work properly but I still get this run on most times. All the hoses are are new, fittings tight etc.
Thanks,
Mike
Mike Crowley

Pat,

You have received a lot of good advice already, but I will share my experience with you. My 74 1/2 "B" had a run on problem, and like yours, the anti-run on valve checked out fine. A few weeks ago, for no particular rerason, I changed out the carbon in the fuel cannister. I also reset the timing and adjusted the carbs...voila!...no more run on. I'm not certain which of the actions fixed it, but it is gone. E-mail me if you would like the write up for re-doing the cannister. I found it on this BB, but don't recollect where exactly.

Cheers, Doug

Doug Keene

Doug,
I just re-filled the canister, too. This morning, I spent some time checking and trying to fine tune the timing. Just got back from a drive, at it seems to have stopped the run-on. Also filled up with high-test, while I was out, so that may have helped.
The timing was actually a bit retarded, rather than too advanced. Not sure whether that would cause the problem, or not - most of the reading I have been doing points to a too-advanced condition.
At any rate, we'll see if it solved the problem (temp here is a bit cooler today, so that might also be a factor).
Still have a bit of popping on rapid deceleration, though. I think that is due to a hole in the exhaust system - just have to set aside some time to get to that.
Thanks for the info,
Pat
Patrick Callan

Pat - a vacuum could develop at switch off if certain pipes were blocked (!), but not normally. In practice the whole system should always be at atmospheric pressure, the only expection being immediately as the ignition is switched off and before the engine has come to a rest. In this condition inlet manifold vacuum is applied to the float chamber(s) via the charcoal canister, but as this is open to atmosphere via the jets the amount of depression developed is extremely small. Gunged carbon could well block the canister, giving the same effect as a blocked pipe.

Thanks for asking the question about the ignition switch, the bit about it having to be on to test the valve is wrong. As I say earlier in the piece it is the ignition switch being *off* that causes the valve to operate in series with the oil pressure switch being on. When the valve stops the engine the oil pressure drops, the oil pressure switch opens, and releases the valve. So the way to check if the valve is operating is simply to ground the oil presure switch wire, the ignition must be off.

Popping on the overrun is usually caused by the gulp valve not injecting air from the air pump into the inlet manifold. Excessive popping is a feature of air injection systems which is why the gulp valve was added. But all systems will pop on a long downhill stretch, and be more noticeable with a hole in the exhaust.

Mike - the system should work exactly the same with a Weber as with the Zenith. It must have the same connections, though, i.e. one to the carb body to evacuate the crankcase, one to the inlet manifold, and one to the float chamber, plus all the other plumbing and parts of course.
Paul Hunt

Paul,
All great info, as usual - thanks.
I didn't drive the car today, but will monitor everything carefully again tomorrow. On Sunday, the timing adjustments that I made seemed to have had an impact on the run-on. I think that had more of an impact than the fill-up with high-test ( I would expect that to take a little longer to respond).
I know I have a leak in my exhaust system, so I'm hoping that is the poping issue. Another "project."
At any rate, I really appreciate the additional info, and I'll keep an eye on the vacuum release when I fill-up.
Thanks again,
Pat
Patrick Callan

Thanks Paul. I will have to wait until my paint job is complete to check everything out.

Mike
Mike Crowley

This thread was discussed between 21/08/2003 and 26/08/2003

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