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MG MGB Technical - Help engine keeps running

The engine on my 79 MGB keeps running after it is turned off.

I just replaced the engine in my 79 MGB. I also replaced the carb with a Webber. I have a Mallory mechanical advanced distributor with a pointless electronics kit in it.

The car runs great. It has a lot of get up and go. When it is cold it turns off fine. When it is wormed up it tries to keep running after I shut it off. The car never get to hot the temp gauge reads in the middle.


Any ideas
Bob

This is an extremely common problem with the B engine, and I don't think there is any guaranteed cure. Try to use the highest octane fuel you can find. Get someone who knows what they're doing to tune it on a rolling road. Then do what most of us do - switch off, then lift the clutch with the car in gear and the brake firmly applied. Works every time!
Mike
Mike Howlett

Bob,

Mike's right "running-on" is very common & happens with mine. I to have used the clutch method - brakes on, clutch up & switch off as the revs drop. MG's "run-on" from new, a professional tune may cure the problem. Common symptoms are high idle, low octane fuel & ignition timing advanced. Try retarding the ignition a couple of degrees as this may help. Also you can buy an anti-run on valve which may elminate the problem altogether. I am fitting one this weekend if you want i'll e-mail you how well this works once it's fitted.

Cheers

Scott
Scott Ralph

Thanks for your suggestions

I just found some other info about the run-on valve but no diagram. This looks like the way to go. The car has had its emission equipment removed. Is it hard to install this run-on valve and what is needed?

Again thanks
Bob
Bob

Bob,

The anti-run on valve is connected to brake servo (which i think you can buy a fitting kit for) otherwise i think it connects to the manifold. If the instructions i get with mine has a diagram i can scan it & e-mail it to you if you want me to. I thought that later B's including your 79 came with anti run on volves as standard, anyone know for sure?

Scott
Scott Ralph

Thanks Scott. That would be great.

Where did you buy yours. I would like to oder one myself.

Bob
Bob

I bought mine from the MG owners club, i live in England not the USA, it's just the BBS won't let me put the correct details! I think Moss sells them & the fitting kit. I will probably recieve mine tomorrow if so i will e-mail the fitting instructions.

Scott
Scott Ralph

Bob,

There is a difference between engine run-on and engine dieseling when the ignition is turned off.

If after the ignition is turned off the engine continues to run normally then there is an easy cure. With late model B's, there is a tendency for electrical feedback to flow through the ignition warning light ciruit. This feedback continues to energize the alternator (and the ignition circuit) when the switch is turned off, resulting in the engine continuing to run. To correct this, go to Radio Shack (or any electronics parts store) and purchase a power diode rated at least at 1 amp. Install the diode in the ignition warning light circuit (it does not matter which wire you choose). The diode is polarized, meaning it has an input and output terminal. If you install it backwards, the ignition warning light will not illuminate when you turn the ignition on before starting. If this happens, simply invert the diode and reconnect it to the wires. (I think that Paul Hunt has an informational page on his web site regarding this topic.)

If you are getting dieseling (heavy knocking of the engine) then you should re-install the anti-run-on valve that was part of your original equipment. You could also try a higher octance fuel or spark plugs with a different heat range while your new engine is breaking-in.
Steven Rechter

Actually what happens is after you turn the switch off the engine knocks and shakes real bad then just before it dies it gives out a big wheez through the carbuator.

It does not do it when it is cold.
Bob

This fault is a common B series engine problem and occurs with MGA's etc as well. It is generally due to the pointed end of the head surface in the heart shaped combustion chamber getting so hot that it acts as a spark plug and continues combustion after the ignition is shut off. It generally does not happen on all cylinders which is why you get the unbalanced knocking and shaking. The best cure is to remove the head and reprofile the combustion chamber shape by radiusing the end of the protrusion in accordance with the MG factory tuning manual.
Phil Stafford

Your 79 has an anti run on valve already installed in the ignition system - in order to check it real quick just remove the ignition warning light and see if the motor shuts down immediately - if it does not you have a mechanical problem (carbon deposits etc.) if it does shut down then your circuit is correct. You can check the anti run on valve by looking in the Bently book - do not use the Haynes as they don't seem to concerned with these small parts. One other thing - you must keep the ignition warning light system as is - this is what triggers your alternator. If you mess with this circuit you stand a chance of blowing the diodes in the alternater - The voice of experience. By the by, I just let mine keep running - it shuts down about 10 seconds as soon as the voltage bleeds off and caused no real problems. Good luck
JAMES NELSON

I have fitted an MGOC anti run on valve. It doesn't seem to make much difference. Be careful where you mount the valve, as mine 'fell off' when the mounting plate came away from the valve (fatigue).

Lowering the idle speed to as low as possible seems to help though, as does running the engine at idle for a few seconds before switching off. Spark plugs make a big difference too (same reason as shape of combustion chamber). The problems is reduced on my car when I use octane booster or the new Shell Optimax (high octane) petrol we've got in UK now.



Neil
Neil

Thanks for all your suggestions.

I filled up with 93-octane fuel and then put an octane booster in the tank. Rechecked the timing and carb setting. It seems to be working better now. But still does it a little. I am still thinking about putting the anti Run-on valve back into the system

Using the clutch works but my Ol’Lady can not seem to get the hang of it yet without jumping forward

Bob
Bob

Are we talking about run-on or diesling here?

It seems to me that with the instalation of a richer running carb set up, and a rebuilt engine in which you probably skimmed the head a bit and upgraded the camshaft, that you could be having a diesling problem. It's due to the build up of carbon getting freakisly hot and increasing the compression and remaining hot enough to continue to drive the engine. Use the high octaine stuff, get your car out on the highway often, lean up the carb a bit, and shift a little later that normal.
Dave

I found a good site that shows how the vacuum hoses along with the Anti Run-on valve is to be connected.

www://mgbexperience.com/service/weber-hose.html


Bob
Bob

My farther sent me this description on how an anti run-on valve works.

Does this sound correct?

An Anti Run-on valve is a valve connected between the intake manifold and fresh air. It is electrically controlled by the ignition witch. When the ignition switch is on the valve is closed not allowing any fresh air into the manifold. When the ignition switch is off the valve opens allowing fresh air into the intake manifold shutting off the car faster.


Bob
Bob

Sounds like your dads spot on to me, thats exactly what it does. Cue everyone adding to this thread telling me i am wrong!

I think the idea is that the fresh air makes the mixture so lean that it can't ignite

Scott
Scott Ralph

This thread was discussed between 15/01/2002 and 17/01/2002

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