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MG Midget and Sprite Technical - Pre igniting

Hello chaps,

I have a small issue with my engine. I think it may be pre igniting. When I switch it off, it occasionally grumbles and rumbles, sometimes its worse than others. The other day it had a real go at starting itself up again!.

So, its potentially a faulty earth connection somewhere. I've also been told that a carbon build up can lead to pre-ignition, but I'm not entirely sure how this works.

Does anyone else suffer from this with their 1500 triumph engine? and whats the solution?

Thoughts appreciated.
Christian
C L Carter

Hi Christian

I think you have to distinguish 'running on' when the ignition is turned off from pre-igniting (or knocking) when the engine is running. You seem to be describing 'running on'. I have had this problem in the past on my 1275 engine.

Basically, something is causing the fuel charge to ignite in the absence of a spark (cos the ignition is off). I would carry out the following:
* make sure the carbs are not running lean (that causes hot running and hot plugs)
* check the ignition timing as retarded ignition can cause the engine/plugs to run hot
* make sure the tick-over setting is not too high
* make sure you have the right plugs (you can go one grade colder to see if that helps)
* try 97 octane fuel or add an octane booster.

In my case I found that an octane booster did the trick and have not had this problem for years.
Chris H (1970 Midget 1275)

I have a 76 1500 midget and the engine used to 'run on'
I use 97 octane, the problem went away
Nigel Axtell

Right, thanks.

So the engine is running on. I also feel its idling too high so I will address that and see if the problem goes away. It idles nicely at 800 when cold but as the drive goes on and the engine warms up it idles ar 1300- 1400... once it was idling at over 1500 (according to the revometer)

I only wish changing the idle stops was an easier job for access!

Cheers,
Christan
C L Carter

Just a thought about the idle speed.

If you adjust it to a lower speed when you have been for a run and find it is idling too quickly then it will not idle when colder, with the choke fully in, but not fully warmed up.

An increasing idle, apart from just warming up normally, can be due to a slight air leak on the induction side (leaking breather pipes, gaskets etc) or the old 1500 engines notorious, for some, sticking accelerator cable which can make getting a reliable idle when hot nearly impossible.

A change of cable can help.
There have been a number of posts about it in this forum.

A too fast idle will definitely contribute to running on.
JB Anderson

Christian,
In the mean time, if you turn the engine off with it in gear, hand brake on and with slight drag on the clutch and release the clutch as the engine slows it will stop dead.
I once witnessed a Spitfire running on in a car park in Enfield Town (the location is irrelevant really). I walked passed it on the way to McDonalds (they were still a novelty at the time). I bought my big mac and chips and walked back to my car. The Spitfire was still running on. I even commented to the yooffs standing around it and they confirmed that the ignition was off and it would be another 10 minutes before it stopped!
I wonder what damage was being done to that engine's internals?
Dave
Dave Brown

Yes I've had throttle issues. I've changed the cable, it made it worse cos it was shorter than the one I had on. So I'm going to change it back. In the mean time I'm going to make my own out of teflon bike cable.

Yes, I was worried about changing the idle because of it possibly not starting when cold. I thought it may start with more choke...

I have also considered poor intake sealing... the exhaust gasket seems corroded and dying and I can't really see the intake gasket... or do they share a gasket? In which case It may be worth changing that.

Cheers for the tip on stopping the engine though. Will give that a try!
C L Carter

Retard timing 4 degrees see if that helps. This will cool the whole combustion chamber. Modern fuel can be less tollerant and it may not be possible to time the car as the manual suggests. See how far away from the recommended timing you are running. I often get this when I've over advanced the timing (looking for more power again..).

A static timing light is ok, but dynamic (with stobe) gives a better inidcation of what's really going on.

To prevent it by the way; Stop, apply hand brake, leave in gear and as you switch off just let the clutch drag a bit. This isn't a permanent fix, but will allow you to arrive at Club meetings on hot days with your dignity intact!

JE
James Eastwood

JE's advice regarding ignition timing is spot on, because when the ignition is over advanced and the engine is switched off, the "run on" can actually be running backwards. Not a good thing. To check if this is happening open the bonnet before you switch off and watch the fan direction.

Bernie.
b higginson

Yep over advanced timing (always the case when some pub mechanic sets your timing for you be ear!!) will result in pinking and pre ignition. And exactly as described will cause the engine to run on when the ignition is switched off. However as Bernie states run on is a poor description because it is actually run BACK and that means the engine runs backwards which means as will the oil pump and thus your engine will be spinning without oil flow!!

When we build high compression engines you can reduce the timing to about 28 degrees without a problem with reduce power, if this does not cure pinking then higher octane fuel is required. If everything else is OK then you can do as suggested and stop the engine in gear.

BL tried to cure this problem on its higher performance engines by installing a valve which opened into the inlet manifold when the ignition was swtiched off. This allowed cool air to enter the manifold and dilute/quench the detonation. In my experience this did not work too well.
Robert (Bob) Midget Turbo

We've recently found and bought a Distributorgraph for the Classic Car Clinic and I'm testing as many distributors as I can... You would be amazed at how many are just not right... The main thing being worn advance springs and incorrect advance curve. Get that right and you'll find you're driving a different car...!

Simply, if both springs in the base of your distributor are loose (the light one should be in tension all the time, the strong one should have a small amount of play), then you will never get you initial timing correct.

Another thing of interest... The centrifugal advance starts to operate at VERY low revs, this is to assist starting.

I could talk lots more about this, but my suggestion would be that most Spridget (and others) should have their distributors looked at by someone with one of these machines. (I know one good guy in the UK if anyone is interested...).

Mark.
M T Boldry

When people are talking about the engine running BACK, when switching the engine off.

Are you saying the engine, stops, and then reverses?

So that instead of running clockwise as viewed from the front, it runs ANTICLOCKwise?
Lawrence Slater

Yes Lawrence. That's why it first makes an awful noise and it runs horribly.
Alex G Matla

I have to say that has truly amazed me.

I've had run on, on many cars including my sprite, and even thought about fitting a valve like the one bob describes BL as investigating, but I truly never noticed the engine run stop, reverse direction and then run backwards.

I have even been under the bonnet at the time, fiddling with the choke and pulling ht leads off etc to stop it, before I had any understanding what was actually causing it.

So either I'm blind, or run on occurs in the conventional direction too, and my engine has never reversed, because I've never noticed that. :)
Lawrence Slater

It is possible that it can do that also mate. I generally dont give it chance :)
Bob Turbo Midget England

I can't understand why or how the engine would stop rotating...and then start rotating in the opposite direction...surely must defy some law of physics.

Is there a you-tube video of such a thing actually happening?
JB Anderson

Yes there is.
Go to Youtube and search for "Nissan Pathfinder VG30 running backwards". It shows someone trying to start a badly timed engine, then it starts to go the other way. There is also another clip of it when it's cured.
Sorry, but I don't know how to put a direct link on here. Can someone tell me how?

Bernie.
b higginson

Surely a 4-stroke engine can only run backwards so long it can pull fuel mix through from the exhuast (that's found its way through to ex manifold earlier cranking on the starter)?

Exhuasting several strokes through the carbs sounds messy (and potentially fire-raising).

I've only ever had an engine fire one cyl whilst cranking at which point I released the key. Sometimes trying to start engines that are over-advanced, the cranking slows as the cylinders fire early, but with only enough gusto to slow down the cranking.

For engines with ECUs, I always set advance to zero for cranking so that there's no chance of the above happening. Frankly I don't see how ignition advance really helps starting... BRB starts much better, esp when hot, with crank-avd = 0.

A
Anthony Cutler

Don't agree with most of that Ant but definately agree that during starting advance needs to be zero or near. Not sure where Mark has got that info from?
Robert (Bob) Midget Turbo

Yup, found it. Amazing.

http://www.myneko.com/Nissan-Pathfinder-VG30-Running-Backwards__rXoYQ_nqbMY.html

BUT, that was running backward for a short time when trying to start the engine. No doubt so far advanced, it was forced into reverse when the ignition was still on.

But I've been searching and can't find an incidence of an engine running in the normal direction, then stopping, and reversing direction when the ignition is switched off.

Running on is caused by sufficient heat in the compression chamber to ignite remaining fuel/air vapour. When the ignition is switched off, the coil field collapses, and the last remnants of spark dissipate into the cylinders. After that only the heat from inside the cylinder or exhuast port can ignite the remaining fuel, or fuel still being drawn in through the intakes.

Once the ignition is switched off, ignition advance is irrelevant surely.

When the ignition is switched on and the engine is running, if the advance is too great, then the premature spark ignites the mixture prior to the optimum time, and has the effect of trying to drive the ascending piston down again. (bakwards). That must be what happens to the Nissan

As anthony said, it will when starting, but not when running on after the ignition is switched off.

I think. :)
Lawrence Slater

Come on everyone, its NOT the engine suddenly running backwards!!

I've had three 1275 Midgets and they were all prone to this. Certainly not helped by 95ron fuel. It's also very temp dependant, ie only happens when the engine is hot. There is something about his engine/head configuration that makes it prone.

As for running backwards, it's not this because the engine is already turning the correct way when the ignition circuit is cut, it just fails to stop. Certainly not stopping and rotating in reverse!

Robin Cohen

Robin. It does, bit not always run backwards. The ignition is switched off, the engine dies then the next piston up ignites the residual charge via detonation because of heat, but it does it before it gets to the top of the compression stroke and so never goes over the top but goes back down the way it came and the flywheel helps with a bit of momentum. If you've never seen it happen I can understand why you would say it can't happen, but I've seen it lots of times.

Bernie.
b higginson

what do you guys dio with your cars !?

I've never had run-on/pre-ignigtion with my Spridgets (B yes)

must be all this lack of servicing and maintence and leaving it to roadside repairs when you break down LOL
Nigel Atkins

lol Nigel,

I'm gonna come round one dark night, and spike your tank with 2 star fuel, just so your in the spridget run-on club. :)
Lawrence Slater

don't bother at the moment it's brim full of Tesco's finest 95 octane, not that 5¾ gallons lasts that long

and where do you find 2* nowadays, you've not been storing petrol since the '70s shortages have you
Nigel Atkins

Nigel, if you've never had any run on, you are clearly not driving it hard enough to get it hot! Use the loud pedal more! Joking aside, I'd really like to know what is getting so hot in the cyclinders, I have a suspision that it's to do with the head gasket fire ring which must overhang a touch. It's not the plugs, I've tried much colder plugs with little effect. I just stop with the clutch in and that seems to stop it. Incidentally, etype (4.2)s do much the same.
Robin Cohen

Nigel. All that Ethanol in your tank! I've been avoiding Tesco since I found out that they are putting it in and not informing people of the fact. Of course, other companies could be doing the same.

Bernie.
b higginson

Christian,
any progress?

Robin,
I think my recent blasts over the Welsh mountains would be classed as hard driving certainly my mate following in the Supra thought so, he's convinced the engine is nowhere near standard

it's modestly just off standard but the whole car is fully serviced and maintained (well mechanically at least) and is set up quite well - so it goes well despite its driver's lack of talent

Bernie,
first time I've put a tankful of Tesco 95 in for some while but when I was up, down and around the NEC for the three days running on Tesco 99 the car went well, in fact another mate following down the M6 said it seemed to pick up very well at motorway speeds

I'm sure the extra octane is more (if not all) placebo, remember I also have the benefit of a fully electronic 123 dissy and Peter Burgess rolling road tune-up
Nigel Atkins

Is there any way to tell if they are adding ethanol? Don't they have to say on the pump or something?
Lawrence Slater

from Tesco's web site

" . . . E10 is on the agenda for European Union member countries and this may become a must-do on unleaded 95 in the future. However, we plan to keep Momentum99 as a legacy grade so that older vehicles such as yours can continue to run."
Nigel Atkins

Tesco 99 specification

max 5% Ethanol including bioethanol

see - http://www.tesco.com/Momentum99/productBenefits-specs.asp
Nigel Atkins

Wish there was a Tesco fuel pump near me. I could get points. lol.
Lawrence Slater

I never see the points my wife using them on nights away and totally unecessaery AA/RAC memembership
Nigel Atkins

This thread was discussed between 22/11/2011 and 27/11/2011

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