MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Engine runs for a bit after ignition off

When the car has been running for a while (30 mins plus) it often continues to turn over for a few seconds when I switch the engine off. I can live with this, but it doesn't always happen which I can't live with! Any suggestions as to why? Thanks
I Sterling

have a look in the archives under RUNNING ON theres loads of info there about it.
regards bob.
trebor

a few quick check items for you:

- is the timing correct?
- is there an air leak around your intake somewhere?
- is there carbon buildup in your combustion chambers?
- are your spark plugs correct heat range?

the cause of running on is a too lean mixture, or hot spots in the combustion chamber causing ignition after the system was turned off

I had a car in which the main things checked out (timing, valve clearances, spark plugs, mixture), but it still did it. Then, one day, I happened to notice the manifold nuts were loose. Doh, there was a vacuum leak in the intake manifold. Fixing that solved the running on trouble.


Norm

Norm Kerr

This is a very common problem on the MGB so have a look in the archives for that board too. Norm has listed th common culprits - unfortunately modern fuel makes things worse with its lower octane rating. To prevent it in the short term before you are able to tackle it properly, plant your right foot on the foot brake, your left on the clutch, engage a high gear, say third, switch off and at the same time gently raise the clutch to stall the engine. Much kinder than letting it shake, rattle and roll.
Mike Howlett

Mine was simply a case of car needing super unleaded. Higher compression and warmer cam doesn't like 95 Ron fuel.
Robin Cohen

My 1500 used to do it. But there was some other variable factor as it was very inconsistent. It would behave perfectly for weeks and then with no apparent changes at all it would start to run on. It would then mysteriously correct itself. I never did manage to isolate what it was that caused it!
Guy W

yepp to all the above...

if this just started

most likely it will be poor fuel, or manifold air leak ...as the most common culprets

if your getting a lot of popping on decelleratiin.as.well then..id.strongly suggest a vac leak

prop
Prop and the Blackhole Midget

As Mike says, very common in a B, not so in a midget.
Norm's list is a good one, air leaks are the no 1 cause - assuming you have the right fuel octane.

Usual air leak sources are worn throttle spinles and in A series damaged manifold gasket where it is thin above and below the intake ports. If the spindles are leaking air, the idle also becomes erratic, so the engine idle speed is usually increased to smooth it. Faster idle then further accentuates run-on.

I've coverd run-on more fully at http://www.mgparts.co.nz/advice/technical-notes/run-on/. It's targetted at B series due to the prevalence of the problem there, but everything is directly translatable to A series.
Paul Walbran

One cause of running on is that the engine is running too hot. Check your car is not running overly hot first - coolant etc - and yes, check all over for air leaks.

If all else fails, try a grade higher hotter plug....Easiest way to frig the run on is to depress the throttle simultaneously when turning ignition off.
Mark O

as well as what's already been put I'd add if it's just started it could be a recent fuel fill up (octane or make up of fuel) and if you don't use your car a lot or constant short journeys then a good blow out run might cure it as it does with many other minor aliments that build because of lack of frequent reasonable distance use
Nigel Atkins

Nigel, that doesn't work, you're just pouring more fuel and air into the chambers and allowing it to run on longer. I know, minne did it for ages. Colder plugs,,not much difference, I ran BP8 plugs and it still ran on when hot. 98 Ron fuel, sorted. The lesson is, detonation in the chamber caused by too low an octane fuel, and the only way to fix is lower the cr or change fuel.
Robin Cohen

Hi Robin, I think we’re agreeing on octane but I didn’t put it well

I Sterling has not put that his is any other than a standard engine and has no public vehicle profile to view so I took it as a standard engine

the Driver’s Handbook (full of load of info so recommended to I Sterling if he hasn’t a copy) has the standard UK car running on 97 octane but you can run some on 95 octane with adjustments

I meant the run-on could be caused by dodgy fuel or lower octane than required for the engine and that many aliments on classics are caused by lack of frequent use so a tankful of the correct octane and a good blow out run might cure a few things

this is in addition to what others and yourself have already put

we also don’t know to what extent the car has been fully service and set up, how far the car went in 30 minutes, 5 miles, 30 miles - so the suggestions will be wide without further information or questions and answers – it’s the usual lack of step by step diagnostics we all usually miss on these threads
Nigel Atkins

Opps. Sorry!

Car is standard(I think). Original engine, electrics, carbs etc. 58,000 listed (158,000? Don't think so?). Original head, running unleaded with fuel additive.

Only currently being used of Fridays whatever the weather is to commute to work. 2 miles to motorway, 22 miles on clear motorway at 50mph (60 max) then 2 miles (5 mins) town driving at the other end. 52 mile round trip. Engine seems to be running well and quite smooth. Starts ok, big hole in rear silencer which I must replace. Occasional run at a weekend, usually quite short (max 10 miles)

FYI this is me;

www.mgmidget.moonfruit.com

Thanks again for your thoughts

Ian
I Sterling

Hi Ian, no problem, now I see you’re Ian with the web site :)

if you want to you can click the Customise button here to include your first name and make your vehicle profile public with photo(s)

the commute is just about right for the car and once a week is good, if you missed the odd week or two it’d still be OK but more frequent and occasional longer use is much better

the weekend run of 10 miles will do no favours for your battery as it’ll not have recovered from starting and the engine will have additional wear and stress from the starting and short use, if you could extended it to 20 miles it’d be better – testing and using your wipers, blower, horn, etc., is a good idea

keep up with the servicing detailed in the Driver’s Handbook but be more frequent with greasing the front suspension, I do mine every 3 months plus just before the MoT to hopefully avoid ‘play in king pin’

if you want I have loads of non-technical, non-mechanical general notes just email me
Nigel Atkins

ETA: for run-on - all that has already been put plus if you're running on 95 octane try changing to 97, if you're already running 97 try running on 99 (Shell, Tesco, ect.) I run my 1275 on 95 or 99 depending on what's available to where I am with the car
Nigel Atkins

This thread was discussed between 05/01/2013 and 11/01/2013

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.