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MG MGB Technical - Engine runs on

Hi All.
My new B engine runs on when I turn the ignition off. Two questions,why and what can ivdo about it?
Thanks
s page

The later B engines were prone to running on. My R/B was very fuel sensitive and with fuel from the wrong source, would run-on. Because the mixture is not receiving a properly timed ignition, the result is quite a rattling banging, worrying noise. The ignition is usually from a particular "hot spot" within the combustion chamber. The small peak between the two valve seats, if it's still there, is one possibility. Burrs and sharp edges left after machining and sometimes plug tips, (colder plugs can help this).
You could try enriching the mixture a flat or 1/4 turn (HIF). Fuel does help cool the combustion chamber, also retard the timing a couple of degrees. See if the higher octane fuel helps too.
Allan Reeling

Until you can find a fix, you can put your foot firmly on the brake pedal, depress the clutch and select third or fourth gear, then as you turn off the key gently lift the clutch to stall the engine. Much kinder than letting it shake and rattle in its mountings.

Why does it do this? It's basically a problem with modern petrol which is nothing like the fuel the engine was designed for. Although you have switched off the ignition, there will be hot spots within the combustion chambers that can ignite the fuel without a spark occuring. As Allan says causes can be too low an octane fuel, too weak a mixture, or too advanced ignition. All three can cause hot spots in the chambers leading to running-on.

If its a new engine, why not go back to the people who supplied it and ask them what they are going to do about it?
Mike Howlett

I find that just putting my foot on the clutch will normally stop it.
Dave O'Neill 2

You need to define what you mean by running on. Also, it wouldn't hurt to know what car you are working on since they changed through the years.

When you say running on, to you mean dieseling, i.e. chugging and erratic, or do you mean that it acts like you did not turn it off.

I don't know if it is true with the British market cars, but with the USA spec cars there is a wiring issue in newer models that can back feed power to the coil causing the car to continue running in a normal fashion after turning the key off.

Charley
C R Huff

Idle speed, if its idling to fast above 750rpm that will not help, if its a new engine as you say, if the idle was set correct at fitment as it has freed off with running the idle speed will increase if you are having a oil change/check over at between 500/1000 miles, get it all sorted then, Tappets, dwell, timing, reset the carbs,etc, I always run mine on shell V power 5%CO 10 degrees static timing 60% dwell 750 idle speed, never runs on
Andy Tilney

I put a Peter Burgess modified head on mine. Never ran on or pinked after that, even on supermarket 95 octane. I never found I could get the idle speed as low as 750 rpm. Probably a combination of worn carbs and distributor. Mine was much happier idling at around 1000 rpm.
Mike Howlett

We tell owners to let the engine idle for 20 sec or so to allow the combustion areas to cool down.
Two main causes we find if they run on. Firstly, idle speed too high, drop it until running on stops. Secondly, too little advance at idle. Mixture still burning when ex valve opens and overheats ex valve. To be honest I have not found too much advance to cause run on just hard for starter motor to get going.
If those two fail to solve the problem you may need to search for a more exotic reason. Or, as Mike says, stall the engine.

Peter
Peter Burgess Tuning

Usually no need to stall the engine!

Just floor the accelerator as you turn off the ignition, the sudden rush of air should kill the engine.

Will also stop it running in reverse direction ....
Chris at Octarine Services

It may not pertain to this type of running on.
I installed an electric cooling fan on a Jaguar XK-140 and it worked well.With the fan turned on and I shut the engine off it would continue to run for about 30 seconds getting slower until the engine finally stopped. I found that this would not happen if the fan was switched off. The fan was supplying voltage to the coil until it became slow enough to stop being a generator.
A simple diode in the 12volt line to the fan solved the problem.

Sandy
Sanders

"Usually no need to stall the engine!

Just floor the accelerator as you turn off the ignition, the sudden rush of air should kill the engine."

That is the exact method I use and no run on. Unless I forget and then it bucks like a mule.
Max71


All, I recently consummated a "trade" for a 1980 MGB. It has run-on issues more often than not. I will pursue the various suggestions one at a time, and hopefully eliminate this "dieseling" nuisance - or, at least greatly reduce the occurrence! Thanks, Jerry
Jerry Chandler 1951 TD

Hate both of those 'manual' methods. I installed an anti-runon valve and had no more problems, just switch off as normal http://www.mgb-stuff.org.uk/fueltext.htm#solution

But the valve I used is now ridiculously expensive, when you can find it. If you can find a generic valve that is closed when powered, and designed to be closed for the engine to run, then that should be fine. Note that the factory valve for North American spec operates the other way round, and is only powered when it is needed to stop the engine.
paulh4

Paul
Don't know if it's the same over your way but here the Mk11 2.0lt Escorts had a nice little valve on them, it was ign. powered and was mounted on the inner guard with it's own little air filter and a hose to the manifold, you could mount/hide it anywhere really-- There should be plenty of them in the wreckers over there

willy
William Revit

Hey Willy, the wreckers have all but gone :(
Peter
Peter Burgess Tuning

Bit the same here Peter,- the scrap metal guys grab everything they can get their hands on to feed the hungry steel recycling industry market
But there are still a few propper old wreckers about
My favourite,is about 40klm East of me and it's real old school, like bring your tools and get it yourself---It's kind of relaxing going in there and roaming round the paddock full of cars looking for 'that' item of interest. - No ,not the snakes, they're easy to find-- It's mostly run by the office girl and she's right on top of it, knows what's what and doesn't have to race off and get prices from the Boss--it works well
willy
William Revit

Willy, Moss Europe stock these anti run on valves see this link:

http://www.moss-europe.co.uk/anti-run-on-valve-
stn127z.html

Fitting instruction here:

http://www.moss-europe.fr/media/pdf/STN127Z_instructions.pdf

They are not cheap at £75.60 plus postage! There are a few on ebay also.

I have a rubber bumper car, but fortunately don't have run on problems.

Andy
Andy Robinson

Here's the right link

http://www.moss-europe.co.uk/anti-run-on-valve-stn127z.html
Andy Robinson

That's the same type as I have, but 50% more expensive now. It didn't work as supplied by the MGOC for fitting to a Tee in the brake servo hose, until I re-plumbed it to work like the North American system which sucks the fuel out of the carb jets.

But then I discovered it worked if connected directly to the inlet manifold with a PCV valve adapter, as here http://www.mgb-stuff.org.uk/fueltext.htm#solution2
paulh4

With lower octane fuels it has been a problem here. SOme engine can have everything right and still run on, while others can everything wrong and be fine.
I think head castings around the spark plugs have a significant influence in the way heat is conducted away from the plugs. This conclusion is based on a number of tests with persistent run-on cars, in which a fresh set of plugs is fitted, the engine started and turned off immediately - no run on. Run it for 20 seconds and the run-on re-appears.

I have covered a number of causes in my web page at http://mgparts.co.nz/run-on/
Paul Walbran

Ditto pinking, some cars seem to suffer more than others as you say, and the causes are similar. On an Arden Heritage Run to Shelsley Walsh which included a run up the hill (untimed!) it was noticeable just how many 70s cars were pinking like mad under the abnormal 'load', including mine, in comparison to earlier and later cars. It can't be more than a couple of minutes to get to the top (admittedly after idling at the start line for a while), but the temp gauge was on the H, where I have never seen it before. I had to leave it idling for a while to bring it back down. Lyndsay Porter back in 1980 covered grinding back the 'pier' between the two valves to reduce compression ratio slightly, as well as the hot-spot, to help control both pinking and running-on.
paulh4

That Moss valve looks very similar to the Escort valve
William Revit

Willy, something buried in my memory tells me that's what it is.
Paul Walbran

Watch it Paul, you will become a candidate for rolling road tuning to stop the pinking whilst maintaining performance :)
Standard Bs are already low compression so reducing CR even more seems an odd way of trying to stop pinking. I think Lyndsay must be talking about pre 18v style chambers for 'pier' grinding. I think this is a throw back to the factory recommended mod of radiusing the very sharp MGA 'pier' between the valves as these were very pointed as cast. The last MGA 1622 head and all the Bs had a much better shape.
Peter
Peter Burgess Tuning

Pinking is easily controlled by backing-off the timing a little, you certainly don't need a rolling-road. The mechanical distributor is a pretty crude device and only generates an approximation to an ideal curve. Spending hours on a rolling-road fine-tuning the springs and weights for an ideal curve is an option, but installing a programmable system such as Amethyst or Stealth is far easier and gives the ability to fine-tune ongoing.

"Standard Bs are already low compression". Not sure what you mean by this, whilst the UK B-series may not be as high compression as some engines it is classed as high compression at 8.8:1 and 9.0:1 in comparison to those for some other markets which are classed as low compression at 8.0:1. Whilst you may not be able to induce pinking by advancing the timing on a low compression - it is more likely to stall the starter first - you can certainly induce pinking on a high-compression. Haynes since 1981 has advised that you can improve the performance over and above factory settings by making small adjustments to distributor position, until just the faintest pinking can be heard (or backing-off just short of that if you are happier). That's even more important with today's unleaded, especially if you choose to use 95 octane, but some struggle to reach book timing even on 99 octane. My roadster engine has never been able to run more than book even on leaded, unlike some previous cars. It's a Gold Seal at +40, and who knows maybe a skimmed head, so the resulting compression ration may be higher than spec and so more prone to pinking.
paulh4

The most common cause of pinking in MGB engines is distributors not performing to spec, especially wear in the centrifugal advance mechanism of the distributor.
We find it is unusual for a B with engine 18V582 or earlier to not have this condition unless it has already been rectified,

As a result the distributor gives too much advance too soon compared with original spec. So while the timing is right at idle, by the time 1500 RPM is reached it is several degrees past where it should be and is often nearly at full advance. The extra timing in the wrong place inevitably causes pinking.

Retarding the distributor will of course cure this, but it also reduces the maximum advance, with a flow-on effect to mid and upper range performance and fuel economy. It also retards it at idle, which aggrevates any run-on tendency.

The other common cause here is people using 91 octane in high compression engines. That is very easy to fix for them, drive 200 metres up the road and fill with higher octane ...
Paul Walbran

The real skill is getting the timing right so you don't lose power at any rpms. I must admit that my attempts at seat of the pants road tuning are very poor compared with the inertia dyno. I can understand the enjoyment of on the road tuning but you still do not know if it is correct unless you record the power and try a little less then a little more advance and see the effects. Just making sure the engine does not pink but has max non pink advance does not mean one has obtained the most desirable state of tune for engine efficiency it just means one has set the timing so it doesn't quite pink and nothing else. If that is your goal you have achieved it.

With regard to CR. If you look at the A series they developed from low cr and got better with age, think of the 10+CR of the MG Metro, they are what I consider HC engines not 8.8 Bs. We run 9.75:1 CR on modded heads up to 270 cams. For 285 track engines we run 10.5:1 ( best pump fuel). So it is not the CR causing the pinking which is why I said Mr Porters detuning the standard low 8.8 CR (to me and most other folk) is a bodge.

As Paul W says, a simple retardation of timing may not be the real solution.

Using a programmable system is easier and much quicker done on the rollers then checked on the road under customers usual driving conditions and environment.

Fuel quality and content has changed too much to go by std curves plus or minus a bit in the 36 years since 1981. Rolling roads still measure power 36 years on and, if used correctly will improve the engine performance or confirm someone has got it right by roadtesting and tweaking.
The argument rolling roads kill engines does not obtain with inertia dyno tuning.

Peter
Peter Burgess Tuning

There we differ. I don't see how tweaking the distributor on a rolling road to get minimal or just zero pinking is much different from doing it on the road - with one exception. And that is, the rolling road may not adequately represent ones local roads. In my experience it's evident that getting it just right for the roads round me, needs a smidgen more retardation for hillier districts. Likewise if I were doing it for hilly territory I could do better in Norfolk. That's the beauty of the 25D4 distributor with the vernier control, it only takes seconds to make a small change. But we will never agree.
paulh4

You misunderstand Paul, my goal is not minimal or just zero pinking it is to get the best performance and efficiency from the engine it is nothing to do with what you set your engine to. It seems you have a target of just not pinking but that does not mean best timing for the engine does it?
The programmable dizzy systems are good. Put in a curve, power test it, add maybe 2 degrees everywhere, power test and review data. If all better then try some more advance till power fades then take it back to best power. If the power fades at any point on initial settings with the extra advance, take it back at that point/s and chase best power by means of retarding. We can end up with a non uniform advance curve which cannot be replicated mechanically. I don't see how this can be achieved on the road? What does show up which may not show on the road if you tune by AFR, unless you data log, is that afr will vary with ignition settings which can throw a real spanner in the works. It is a matter of play with fuelling, play with timing and juggle both to get best results. Do you measure or record afr when road testing?
Peter
Peter Burgess Tuning

Paul W
Have to agree with you here
The number of Lucas dizzies that I've found with rusty/stretched or broken springs in the past, makes me think there's probably no old unopened ones out there that are working properly anymore, and if there are the curve probably isn't correct anymore anyway
Paul H
Peter is being very polite to you here-
He knows exactly where the ultimate ign timing should be by extensive testing and I agree with him 100%
The correct method is to run the least amount of advance possible without reducing horsepower for any given speed/load
The method of tuning by getting to the pinging point and backing off a tiddle is old school thought and is fraught with danger
You might get good fuel economy if driving around on hardly any throttle all day but under any sort of load it will be overadvanced and prone to engine damage
With the arrival of ethanol in the fuels now it's even more of an issue as the engines tend not to display audible pinging as much with this fuel--This would mean if tuned by your method they would be running extremely over advanced
With all due respect
willy
William Revit

Willy, no problems experienced in using my method since the 70s, including my roadster and V8 over a combined 50+ years and 200k miles. Same as with mixture, the lifting pins and Mk1 ear have always been fine for me. YMMV.
paulh4

My Mk 1 ears, and I'm sure I'm not alone here, find it almost impossible to hear pinking, over and above exhaust, wind and engine noise. I'm completely with PB, seat of the pants tuning gets you so far, but rolling road gets you further.
Allan Reeling

This thread was discussed between 29/08/2017 and 02/10/2017

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