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MG MGB Technical - running on

I have a 72 B and the engine was rebuilt 4 years ago. Until recently it was running just fine but now every time I stop the engine it "runs on"or some call it "dieseling".
Anyway I havnt investigated yet for the cause but thought before I do I would seek advice as to what items to look at. Seams someone always comes up with something I didnt think of and it helps a great deal. All ideas would be appreciated. regards Brian
brian

Brian

Check you timing. Could be something as simple as the bolt on the distributer getting a little loose and the timing moving.
Bruce Cunha

Hi Brian.

There are various possibilities, but my first suspect would be that there is some 'coke' in the cylinders which is glowing and igniting the fuel / air mix.
In my experience modern high-detergent oils do cause coking in older engines that burn a little oil even when they are in good shape.

Does anyone else have experience of this ?.

Don
Don

I tend to go with Don on this one. And idling too fast. I have no opinion of detergent oils causing coking, tho. The ignition is turned off, the plugs aren't firing, so timing is moot (in my mind).

I think it's kinda cool when the engine runs on, wierd when it runs backwards, tho. I just let the clutch out (with a foot on the brake, too). A couple times last year, I may have had a larger deposit in the combustion chamber than normal. When I shut the car off and got about halfway into the house, BAM! Like a firecracker going off in the exhaust. Super cool!!

I watched a mechanic friend pour water down the carb of an old lady's car once. Amazing how many big pieces of carbon came out the exhaust. Maybe I'll try using a squirt gun or something on mine sometime, can't pour it in from a Coke bottle.
Tom

"Could be something as simple as the bolt on the distributer getting a little loose and the timing moving" or the points rubbing block having worn down and changing the timing. A retarded timing will drop the idle and in an attempt to restore the idle speed, the idle screws are run down a notch. This keeps the throttle disks open further during shut down, allowing fuel to be suked into the engine. Good luck - Dave
David DuBois

Try using high (98) octane fuel such as otimax. If that doesn't help (it should) then look at timeing. If that doesn't work you can put on an "anti run on valve" or a pair if you like. They had them on the american MG's so there are plenty around and are very easy to pumb in. Infact your haines manual will have diagrams. Essentially these small electric valves are connected to the intake manifold (one or more of the blanking bolts) and are closed when the ignition is on. Switch off the ignition and they open up, bypassing the carbies. I suggest you connect them to your air filters.
Range rovers also have them.

Be very wary of putting water near the carbies. Water doesn't compress and you can destroy your engine.
Peter

Engines of that era were always prone to running on and it got much worse with the change to unleaded fuels. I didn't find super unleaded, even Optimax, made much improvement, and whilst reducing the idle speed does help you can end up with a very rough idle and a tendency to stall stuck in traffic (with HS carbs anyway). And whilst retarding the timing reduces the increased pinking with lower octane fuels it also saps performance and increases running temps. North American cars got the anti-runon valve with the 1973 model year and this completely cures it. Retro-fitting that to an earlier model is a bit messy with replacement ignition switch or adding a relay, adding an oil pressure switch, additional wiring and plumbing. I found an after-market valve available in the UK, which just opens and dumps air in the inlet manifold, to be completely useless on its own, but I converted it to work very similar to the North American with a bit of extra plumbing and it has completely cured it on my UK 73. So much so that I can now have the idle at about 800 or 900 rpm, it's smooth, and it doesn't tend to stall in traffic. More info at http://www.mgb-stuff.org.uk/wn_fuelframe.htm and click on 'Running-on'.
Paul Hunt 2

Brian, Don and Tom there is a huge amount on this subject in the archives but you have to be a member to access it. Membership is free so I would suggest you sign up now and have a look.
Iain MacKintosh

Dump the clutch in gear when shutting off and that will stop the problem. Too much carbon or a hot spot in the head.
Mike MaGee

Yes and give the whole transmission a shot up the
a--e at the same time. I've done that as well but don't recommend it.
Iain MacKintosh

This doesn't really help answer the question but mine used to be quite bad for running on but in the last couple of years it's all but stopped doing it, the only two things that have changed are [a] I've gone back to points ignition from electronic, when I did this conversion the timing was obviously not going to be exactly the same and [b] unleaded petrol has less sulphur in it now than it had a couple of years ago[in the UK anyway].
If you are going to experiment with an anti-run on valve it is important that it is fitted to the manifold or as near to it as you can get or it doesn't work very well.
Ron
R. Algie

A lot of talk has been done about the anti-runon valve. But your car ran okay without one up to this point, didn't it? So there's no reason you should have to fit one all of a sudden.

I agree with David DuBois - your idle screws are probably keeping the throttles too far open at shutdown. As he says, this was probably done in response to some other issue which caused the idle speed to drop.

Before anything else, tune up your motor and then set the idle to spec (850 or so?). I'd bet you a doughnut it will run AND stop like a charm.



Matt Kulka

This thread was discussed between 28/09/2005 and 30/09/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.