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MG MGB Technical - Engine running on

The engine on my roadster sometimes runs on for a few seconds after it is switched off, more often when hot. I thought this was caused by the ignition timing not being correct but I cannot remember if it is because it is to far advanced or retarded. It has been set at 14 deg. btc, by strobe. 18v H.Comp. engine, I think this is correct for this engine. any suggestions?
Trevor Harvey

What 18V engine spec have you? 14 degrees btdc at 1000 rpm sounds high. I have an 18v high comprension engine spec 846F. Look on Paul's site under
http://www.mgb-stuff.org.uk/ignitiontext.htm All the specs can be found there.

/Moss
Moss

B's were always renowned for running on and sensitivity to some makes of fuel. Running on is caused by hot spots in the combustion chamber continuing to ignite the mixture, not timed of course hence it's rough nature. These hot spots can be; carbon, rough casting, sharp edges, too hot a grade of plug. Overheating of these spots isn't helped by too weak a mixture or too advanced ignition or too high a compression ratio, all of which increase combustion chamber temps.
14deg might be too advanced on modern unleaded, it could be pinking which won't help combustion chamber temperatures.
Assuming you haven't got air leaks which are weakening the mixture, wind a flat on the jets to richen the mixture and knock 2 deg off the timing. See if it helps.
Allan Reeling

If the idle speed is too high it will add to the running on problem. Set the idle to no more than 750rpm when hot and this should virtually eliminate the problem.
Iain MacKintosh

Depends on the dizzy fitted - early 18V engines are 13 degrees at 600 rpm later ones 11 degrees at 1000 rpm.

Try retarding the advance by a few degrees - unleaded fuel need a couple less than standard anyway so I would aim at 10 degrees at 1000 rpm.

Easy cure for running on - floor the accelerator as you turn off.
Chris at Octarine Services

It is an early dizzy and the info. I have from the Haynes manual and from the MGB-Stuff site recommended by Moss, timing by strobe should be 14deg BTC at 600 rpm. This is what the engine was set up at, but it is running on so I will knock it back a couple of deg. as suggested by Chris and see if it helps. Trev
Trevor Harvey

Trevor,

Try running your engine on 97 octane fuel I'm sure that will cure your problem also you will be protecting your fuel system from the destructive effects of ethonal additives.

Patrick
P.T. Tighe

X2 Patrick.

Or even 98 or 99 if you can get it. MGBs with high compression engine (UK spec) were built to run on the old four star petrol, 98 octane)
M Ratcliff

Running-on with my 73 got much worse with the change to unleaded, even 97 and 98 octane. Certain eras of engine are more susceptible, and to pinking on unleaded. A high idle does make it worse, but lowering mine made it rock and shudder, and stall if left to idle for only a few moments. Retrading the timing makes it worse as the engine is less efficient and runs hotter, as well as reducing performance and economy. In the end I fitted an anti-runon valve - http://www.mgb-stuff.org.uk/fueltext.htm#runon
Paul Hunt

" In the end I fitted an anti-runon valve "

Cheaper to just floor the accelerator - has exactly the same effect.
Chris at Octarine Services

The most common cause of run-on (assuming the octane rating of the fuel is correct as noted above) is air leaking through worn throttle spindles. This accounts for 90% of cases and has a double wammy effect - not only does it lean the mixture out (not good for run-on), the irregularity of the lean and variable mixture necessitates a higher idle speed which further provokes run-on.

As we've historically had rubbish/lower octane fuel, B engines in NZ have always been more touchy than most in this respect. I have a more detailed page on my website on run-on at http://www.mgparts.co.nz/advice/technical-notes/run-on/
Paul Walbran

"Cheaper to just floor the accelerator - has exactly the same effect."

That's horrible, as is stalling it in 4th, and it didn't work. It's *having* to things like that which give classics a bad name.
Paul Hunt

Giving Classics a bad name?

The fact that parts prices start at fifty quid and go up to the stratosphere should give modern cars a bad name...

The fact that most stuff can't be fixed without a huge dealer or garage bill should give modern cars a bad name...

The fact that low profile tyres cost the earth should give modern cars a bad name...

My '64 MGB cost me £200, was rebuilt for £2,000 first time around, then cost £2,500 (ball park) in repairs & parts, followed by another rebuild for £9000... over a 30 year period & 50,000 miles. I've lost more than that in depreciation on one new car! OK so for 17 years and 80,000 miles I owned an MGB GT too which cost me about another £5,000 overall, but I also had three other MGs which in total made me around £8,000 profit.

£10,700 for 30 years motoring? A pound a day plus petrol/oil & A HELL OF A LOT OF FUN should give classic cars a bloody good name, whether they run on or not.

P.S. my MGB runs on like crazy... and long may it do so!
John Prewer

Our experience with running on.

Anti-run on valve only effective if idle mixture so weak the engine hardly runs. Can make the problem worse with OEM mixture setting.

Idle speed needs to be as low as possible while maintaining idle speed. If the thrust washer clearances are poorly set the engine may stall when clutch depressed needing higher idle speed to cope therefore more liable to run on.

Retarded ignition timing fatal, the mixture is still burning merrily when the ex valves open causing overheating of the ex valve.

Don't forget decoking may be needed on a more regular basis with leadfree fuels. Combustion deposits worse as many MGBs don't do many miles and clean the combustions area.

Poor spark plug condition does not help. We notice plugs need the combustion deposits removing on a regular basis. try a colder grade of plug eg NGKBP7ES instead of NGKBP6ES.

As Nigel Atkins would say....regular maintenance!


Peter
Peter Burgess Tuning

I never doubted that what Peter said would work, but it works brilliantly - floor the throttle as you turn the engine off - no running on.

I would be trying to get the engine stopping without running on, but the original engine is going back in soon and the 18G in the car will then go into storage for a while so there doesn't seem much point.

John.
John Prewer

Hi John

Twas Chris that said floor the throttle not me. I would be wary incase the engine decided to pick up though!

Peter
Peter Burgess Tuning

Peter,

Sorry... Chris' advice works but as I said the car will hopefully not have too many trips before the original engine goes back in.

I'm on a big "get it right" trip at the moment having had the car rebuilt for the MGBs 50th and looking forward to its own 50th birthday next 28th Feb. I may even try to take it and show it to the original owner on 4th April which will be the 50th anniversary of its sale.

Unfortunately Hallens of Cambridge who sold it is now a block of flats having been an MG dealer for 40 years...

John.
John Prewer

I've never had one pick up - I guess that if the mixture is OK to start with then it gets weakened so much the engine just dies. If there was excess fuel in the manifold that would sustain firing with the sudden surge of air then the mixture would probably be too rich to sustain autoignition once the spark stops.

Anyway it is a practical approach to dealing with the odd engine that doesn't respond to tuning/maintenance and IMHO is no more "horrible" than having to pull the choke out to start the engine in the first place!

Chris at Octarine Services

"Anti-run on valve only effective if idle mixture so weak the engine hardly runs."

True with the MGOC valve when teed into the servo hose, which is how it is supplied, as that drastically reduces its effectiveness. But the way I use it i.e. like the North American system where it sucks the fuel out of the carb jets, or if plumbed directly into the inlet manifold, works perfectly well with the correct mixture.
Paul Hunt

This thread was discussed between 21/06/2013 and 27/06/2013

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