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MG Midget and Sprite Technical - running on

I have a slight running on problem after a spirited run took all plugs out and the don't seem to bad going to try a cooler plug see if that helps any other suggestions


mark heyworth

I would check the carb balance and make the carbs a bit richer. Those plugs look too lean, especially the one on the right- if that is No 4, the rear car is running too lean.

If that makes no difference, I would also retard the ignition a degree or two and see if that makes a difference.

But only change one thing at a time to see what the effect is.

How high is your idling RPM?
Dominic Clancy

idling at around 1000 rpm
mark heyworth

Air leak on rear carb spindle?

Try spraying carb cleaner or similar flammable spray around the spindle bearing. If revs increase, it is sucking in air which will be weakening the mixture on that carb.
GuyW

hope not new spindles and bushes fitted 3000 miles ago.
mark heyworth

Ok, perhaps not that. How about a manifold air leak then?
GuyW

Mark, I would go with what you said yourself in your original post, "cooler plugs". It can't be ignition timing because you have turned it off. The hot plugs are continuing to ignite the mixture. NGK plugs run cooler as the number gets higher, so if you are running an NGK BP5E, try a BP6E etc.
P.S. mixture looks about right to me.
Lindsay Sampford

had BP5E in trying BP6E will see if its any better it only happens if you drive it hard.
mark heyworth

In the Good Old Days of leaded petrol, a little bit of incandescent carbon would give the same running-on problem. The answer then was a de-coke, remember that routine procedure? The good thing; probably the only good thing, about unleaded petrol, is that it doesn't lay down anything like the same amount of deposit, and combustion chambers stay pretty clean.
Lindsay Sampford

It can be timing, because if the timing is too advanced the car will run hotter and when turned off the extra heat after a hard run is just enough to keep the engine dieseling. Mixture being too lean has the same effect, which is why I suggest both adjustments. The cooler plug will probably help too.
Dominic Clancy

Most common reasons we have for running on are; idle speed too high, weak idle mixture and retarded ignition timing at idle. Most uncommon someone skimmed/milled the head and didn't deburr the chamber to head face sharp edges NA for ally heads but we deburr them out of habit and to be on safe side. We tend to up the plug spec for detonation at revs but I am sure it must help at idle too.
It can be good practice to let the engine idle for say 20 secs or so before switching it off, especially after a hard thrash
Lindsay it can be the effects of ignition timing. If the timing is too retarded the mixture is still burning when the ex valves open and get overheated to the point of causing running on.


Peter
Peter Burgess Tuning

Plenty water in the radiator just a very long shot!
Eddie Cairns

Another thought... seasonal fuel change over for summer blend ...

I tend to notice this in spring most often on my truck, they release the summer fuel and the weather is to cool still and the truck runs a bit odd till the weather warms a bit more
1 Paper

Considering you just rwbuilt the carbs...id retorqe the carb and intake manifold hardware...if the new gaskets where payen or thicker,,, they may have seated in thus leaving the hardware a bit loose

Also... make sure your exhaust pipe is still attached to the braket on the transmission housing...if not the various movements in the exhauste pipe can loosen the manifold hardware tbe intake manifold to leak a bit of air

The key is you said this started right after a hard run and you just rwbuilt the carbs, so id discount that the timing had changed unless the dissy was loose ... and the plugs dont look all that bad to me to cause running on starting after a hard run... so id lean towards air leaks

But id also check your anti frezze level also and make sure your heater matrix water shut off valve is fully open, esp if your really pushing the engine . If you shut off the tap the water does not cool the #4 cyl very well as the water is mostly by passed making #4 run a lot hotter

Prop
1 Paper

Prop could have a point with the fuel as i had to fill at a supermarket with non super unleaded normally use esso synergy +97 or Tesco momentum 99
mark heyworth

Mark
You said you had NGK BP5 plugs and would think about going colder to BP6. I think BP5 is too hot anyway as BP6ES is the standard plug spec (equivalent to Champion N9Y). I use NGK BP7S in my midget and it runs well and no running on for years.
Chris Hasluck

According to NGK, BP5E is standard for a 1500 and BP6E for a 1275.
Dave O'Neill 2

Mmmm! 'retarded ignition timing at idle'

Until recently our 1380 with a 276° cam suffered from run-on occasionally, ever since we first built the engine nearly 20 years ago, even with an MG Metro anti-diesel valve fitted.

In late 2016 I fitted a NODIZ, 3D mapped ignition module to replace a failed Aldon Ignitor unit (after ~17 years and >50k miles, one of the wires connecting the module inside the distributor failed, so not really a problem).

Being fully mappable, I have set the idle advance up to ~25° BTDC, but with any load or engine speed increase, it drops back instantly to ~12/15°.

The idle quality is far better with the additional advance than ever before.

It has not yet been on a rolling road since fitting the NODIZ unit (well overdue for an appointment with Peter B!), so have not been too adventurous with the advance curve, but no matter what driving I have done, including track days, turning off the ignition stops the engine instantly. No hint of run-on, ever.

I have put it down to the cautious(ish) advance curve, but maybe the advance at idle has more to do with it?

Getting back to the topic of plugs it has NGK BP7ES fitted, gapped at 0.035" - never given them a moment's thought, other than checking the gaps and replacing them occasionally.

Richard
Richard Wale

I would put money on low octane fuel, particularly if you are running with a high c/r
Graeme Williams

X2 on Graeme, on normal unleaded my 1500 always runs on, switch to super unleaded running on goes away
timmyk

X3 on Graeme for me also 95 octane cures the issue.
My triumph 2500S has an "anti run on" valve which cunningly applies manifold vacuum to the float bowls when the igmition is turned off. This immediately pulls fuel back from the jets, this is my understanding anyway of how it works.
If this is disabled it will run on horribly.
I have toyed with the notion of fitting a fuel cutoff solenoid to the Midget but will be selling the car soon so have lost the incentive
Cheers
Rod
R W Bowers

Plugs made a bit of difference just a slight cough now so will fill with better fuel and see what happens.
mark heyworth

Mine also runs on occasionally - usually at elevated temperatures after a blast. The sticking plaster approach is to simultaneously press the throttle pedal when turning the ignition off. If running on happens as a matter of course, the root cause is usually the engine running far too hot - for a plethora of reasons but perhaps check for incorrect ignition timing or a weak mix and of course coolant levels. Worn bores/rings are also a possibility.
Oggers

My Mk4 ran on for about 15 seconds once - due to drawing oil up from the timing cover into the manifold and dieseling on the mix.
Graeme Williams

If yyou wwnt to cure the sympton and not the problem.. just put in gear and let off the clutch with the foot of the brake

Prop
1 Paper

Mark, listen to Peter B, he hits the nail on the head with everything he said. Main thing I would add is that worn throttle spindles leak varying amounts of air and you have to raise idles speed as a result to get a clean idle.
Fuel octane is also an issue, if your engine needs higher octane then use it. It is actually cheaper to run in the long term if this is the case, as the burn is much more efficient.
Finally ignition timing has another inderect effect: too retarded and it needs more throttle to maintain idle. More throttle = more fuel to self ignite. We have solved many run-on case, latest one today, by bringin the advance up at idle but retaining normal advance at higher RPM.
Paul Walbran

My old 1300 Escort ran on so badly that I routinely stopped it by stalling it against the brake in 4th gear. The reason was there was so much oil getting into the cylinders down the valve guides when it was stopped. The carbon deposits were enough to ignite the mixture as Lindsay said earlier, but from burnt oil and not from leaded fuel. In fact I think poor oil sealing was always a bigger contributor to carbon head-fouling than fuel ever was.

I ought to have decoked it but I couldn't be ar*ed and just lived with it.

Reason I mention it is the OP photo seems to show a bit of carbon around the ends of the plugs. If the engine has been unobtrusively burning oil for long enough (as my 1500 has) it just might have enough carbon in it to cause a little bit of a run on.
Greybeard

I don't think carbon is a problem had head off 2000 miles ago and looked in with endoscope when i changed plugs.



mark heyworth

Ooh that's lovely Mark!

Sorry - I didn't mean to imply that your engine was coked up. Just that I've seen that happen before.

I keep meaning to treat myself to an endoscope.
Greybeard

I got a usb one off ebay works fine for what i need it for
mark heyworth

Back in the 70s the company gave me a new 1.3 Marina. After a year it ran on so badly I could switch off, get out, lock the car (no central locking), and be inside the house before it stopped. Leaded fuel then of course.
L B Rose

Hey mark

Im on my phone so pic are jot to clear at times

But are all the valves matchinug? SOme look like pual ivey and others appear to be high speed stainless racing
1 Paper

This thread was discussed between 01/06/2017 and 09/06/2017

MG Midget and Sprite Technical index

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