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Magneto engine running on

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Magneto engine running on
Pugwash Offline
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#1
29-11-2018, 08:50 AM (This post was last modified: 29-11-2018, 08:52 AM by Pugwash.)
Hi All,

I have a magneto engine with the later high compression head (post 1937) in my special. I'm pretty sure the pistons are Renault 4 but I didn't build the engine originally so I'm not 100% sure. I've just had the head off due to a blown head gasket and have started it for the first time. I had the head skimmed to ensure it was flat before putting it back. It now runs on when it's switched off and continues for some time until it runs out of petrol in the bowl (SU down draft and electric pump). It switched off fine before. Can anyone suggest what might be causing this? I didn't touch the magneto or its wiring whilst removing and replacing the head.

Thanks in advance.

Paul.
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John Cornforth Offline
Sevenner since 2002
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Location: Hampshire UK
#2
29-11-2018, 09:55 AM
"Running On" is normally used to describe an engine that runs for a few seconds after switching off the ignition, due to something in the combustion chamber staying excessively hot - enough to cause ignition without a spark at the plug.

However, in your case it sounds more like the ignition is somehow continuing to work.  I confess that I'm not that familiar with A7 magnetos, but I would expect there to be some kind of kill switch that disables the spark when you switch off. I suggest you double check that you haven't disturbed the wiring in some way during reassembly, e.g. leaving off an earth connection.
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John Mason Offline
Lifetime lover of car tinkering
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Joined: Dec 2017 Posts: 1,130 Threads: 66
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Location: Nottinghamshire. Robin Hood County
Car type: Austin Ruby Mk1 1935
#3
29-11-2018, 09:56 AM (This post was last modified: 29-11-2018, 10:00 AM by John Mason.)
Paul, Have you got the correct head gasket fitted? I ask because in cases like yours when the engine runs on after switching off and therefore fires without an electric spark it usually means there is something glowing red hot in the combustion chamber igniting the fuel. If you have the wrong gasket it is possible that a part of it is extending out into the chambers and being hot igniting the fuel. Each firing keeps it hot and keeps it running on.

John Mason

P.s. I have just read John's thread above and have to agree that he may be right. Does the engine run on lumpy or normally as if you have not switched off?
Would you believe it "Her who must be obeyed" refers to my Ruby as the toy.
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Ruairidh Dunford Offline
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#4
29-11-2018, 10:18 AM
It does sound like the earth connection to the Mag. is not doing it's job properly, I would look there first...
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Pugwash Offline
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#5
29-11-2018, 10:37 AM
(29-11-2018, 10:18 AM)Ruairidh Dunford Wrote: It does sound like the earth connection to the Mag. is not doing it's job properly, I would look there first...

Thanks guys. I'll check out the earth connection to the magneto. Perhaps I inadvertently disturbed it when working on the engine.

Regards
Paul.
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