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MG MGA - Plugs and running on

I'm a fair weather driver, so am looking forward to Spring. As soon as the weather gets a few degrees warmer, I will start the annual service.

So I was thinking about my spark plugs. I have BP7ES plugs on board that are in reasonable condition and have new NGK BP6ES plugs ready to fit.
I understand that the BP6ES is a hotter plug - my car suffers from "running on" in the warmer weather, so my question please is would fitting warmer plugs aggravate the running on further?


Graham V

I used BP6ES plugs in my 1850cc engine for years Graham with no "running-on" issues.

I now run a 1950cc engine which also ran with BP6ES plugs and no running on.
I have recently switched to the cooler BPR7ES after l had a head gasket failure, to see if a cooler plug would help.

In theory the hotter BP6ES plugs will cause the combustion chamber to run a little hotter around the area of the sparkplug and so this could actually make your running-on problem worse.

The usual suspects are carbon deposits and also a weak mixture which can make the engine run hotter.

You could try an even cooler plug Graham?

Colyn Firth

Grahame, A cooler plug provides a greater area of contact between the insulator and the metal plug body, hence the electrode runs slightly cooler. This may help "running on" if indeed the plug tip is the cause of the ignition. Other causes include; incorrect timing, carbon deposits, and other "hot spots" in the combustion chamber. Because the incoming fuel helps to control combustion chamber temperatures ,too weak a mixture can cause running on. In hot weather, the fuel is also hotter so does this job less well.
Too cool a plug can cause plug fouling, especially on a old engine.
Allan Reeling

Thanks for the comments. I am still a bit confused.
If I have it right, if I run hotter plugs, that might possibly make the running on worse.
But on the other hand, if the running on is caused by carbon deposits, hotter plugs may burn that away and reduce the running on.
Of course as you imply, the chances are that running on may not be significantly affected by the type of plugs I fit, but could well be influenced more by a small adjustment in the timing, etc.

Ideally, once the weather improves, it would be simple to just play around with plugs, timing, etc and see how the running on changes. The thing is, sometimes I can drive for an hour or so, on a hot day and no running on, and the next day, I have the opposite.

Graham V

Yes, it is possible that fitting hotter plugs may increase the running-on because you are making things even hotter around the nose of the spark plugs.
I dont think hotter plugs will "burn away any carbon deposits", most likely they will make them "glow" even more and worsen the problem.

Only a de-coke will get rid of carbon deposits but for the sake of a few pounds, you may as well try a set of COOLER plugs first.

If that improves things, well it was worth the expense, if it doesn't, then next check the mixture and the timing etc.

If there is still no improvement, then I would take the head off and remove all the carbon on the piston crowns and from the combustion chambers.

Colyn Firth

Just to throw a spanner in the works, I used to run Motorcraft 12's in the elan for speed events, they are fairly cold like BP8's or 9's and it would run on like nobody's business. but when I put the 22's back in for the road-,similar to BP7 roughly, it wouldn't look like running on-
Back to front but that's what it did

The only thing I can think of in your case is that if you fit the warmer 6's the engine 'might' idle a bit stronger/smoother and you might-might just be able to run the idle speed down a bit slower which could reduce your running on problem
Also if your idle mixture is on the lean side this could be causing your running on as the warmer the engine gets with a lean idle mixture the faster it will idle and the more likely it is to run on
You need to aim for a good strong 12 1/2 :1 mixture at idle

What is your idle speed now- is it too high anyway
William Revit

Thanks Colyn and Willy

As soon as the weather gets warmer I will start tinkering. The running on has never really bothered me too much, other than it just seems wrong.
It was just that I was trying to decide when I do the annual service, if I should bother with the new plugs I have or leave things alone. Its not really a big deal either way, but I was just curious and was interested to hear some views.

Graham V

I seem to recall that opening up the plug gap slightly can reduce the tendency to run on.
John Bray

When I come to the end of the journey I usually let the engine stabilise at its 800RPM for about 10 secs before switching off. That all said, since I recently got the new jets and finally managed to tune the carbs to my liking the run on tendencies seem to have gone away. Another trick I found that worked was to idle with first gear selected and brakes on. As I switched off I dragged the clutch very slightly.

Steve Gyles

MGBs sometimes have a tendency to run on. Just putting your foot on the clutch as you switch off is often enough.
Dave O'Neill 2

Dave, I assume you meant taking your foot off the clutch and so "stalling" the engine.
As well as the standard causes, Barney's site also mentions electrical running on, when the control box contacts don't release. That would be an easy fix, and I assume the simplest way to test for it might be to turn on something (eg the indicators) before you switch off, and see if they also "run on".
Not relevant to the many who have an alternator of course.
Graham V


No, depressing the clutch. Possibly the extra drag is enough to pull the tickover down slightly.
Dave O'Neill 2

I used to have an MGB which I could not stop from running-on. I even tried changing the plugs to '9s' and it still ran on, so very cold plugs didn't work for me.
With my MGA1600, I decided these modern fuels must need a richer mixture. While we get the tick-over just right on the standard No.6 needles, what happens at speed on these fuels? My engine got a lot hotter at speed with water temp rising higher above say 50 mph and an exhaust sound of a lean over-run on de-acceleration. It could be lean at speed...
I then thought with a richer needle you would still set up a nice tick-over and presumably things then get richer at higher through-puts and so hopefully cooler.
Needles are cheap and the originally recommended richer needle is the 'RO' - so I got a pair and whipped them in, ensuring a nice slow tick-over when hot.
I don't know what these needles do for consumption and it doesn't matter with our low mileage use but there is certainly a lot more power and now runs cooler at speed, with yes, much reduced chance of run-on.
To really kill it dead after a run, before shutting off I let it tick-over for up to 30 seconds and as you switch off just dip the clutch. Try and not smile as it stops dead.

I love your last line Pete, made me smile. "Try and not smile as it stops dead."

As you have already experimented with cooler spark plugs to solve this problem, it would make sense for Graham to give your richer needle option a try.

Let us know afterwards Graham if you actually do smile when the engine "stops dead". :-)

Colyn Firth

Understood - all interesting stuff.
I wont hold back my smile if it works!
Graham V

This issue isn't new. In the early 70s when I was doing around 30k miles a year in my 1500 I spent lots of effort trying to stop it running on. One day I read an original Motor or Autocar road test and even the road test car supplied to them by MG ran on so I gave up worrying. I suppose this confirms running on isn't a side affect of modern petrol.

Paul Dean

Am I missing something on the depressing clutch to increase drag? I said in my post that I have in the past engaged first gear with brakes on then slightly dragged the clutch when I switch off. Others have said to depress the clutch to increase drag. I can't get my head round that one so can someone explain? My thoughts are that with the drive train connected in neutral there is a bit of drag through the whirly bits and swishing of oil in the gearbox. If the clutch is then depressed that drag is released and the only drag is then through the clutch thrust bearing. I would have thought that drag would be less than the gearbox?

Steve Gyles

Good question Steve - you are dead right - I don't get it either but it does seem to help... Maybe there is more drag on the clutch's dry carbon thrust washer than a hot oily gearbox???

It isn't just the friction of the release bearing, you are exerting enough force on the clutch cover to compress the springs/diaphragm, which, in turn, is forcing the crankshaft against the thrust washers.
Dave O'Neill 2

My 79 MGB drops 200 RPM when I depress the clutch prior to shifting into first gear. So yes, the depressing of the clutch does create a "drag" on the engine which may solve your "running on" issue or at least reduce it.


79 MGB
gary hansen

MGs are notorious for having built in hot spots (the tip of the heart shaped chamber) and when they carbon up, usually from running them too rich or keeping the choke on too long, you'll never get them to run properly unless you decoke them and round off the 'Weslake tip'.

A clean stock engine should run on BP6ES. I ran BP8ES on the mild engined race car with no issues.
Bill Spohn

This thread was discussed between 01/02/2018 and 11/02/2018

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