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Magneto Problem 1924 Citroen


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Here's a problem that is stumping me. On the test stand, I set the timing of my 1924 Citroen 5HP with the plugs withdrawn from the engine, but there is no spark when the plugs are screwed in place. The Bosch ZA4 magneto was just serviced by a reputable shop , so I know the magnet is charged; the condenser is new, coil re-wound, etc. The plug wires test OK for continuity. As I say, it does fire the plugs, but only outside the engine. 

 

I have driven the car with this magneto, though it was years ago. This engine runs on magneto only; no battery ignition. Any ideas?

 

Phil

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Edited by MochetVelo (see edit history)
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How do you know you have no spark when the spark plugs are screwed into the block? 

  • If you have spark from a disconnected plug wire held close to the engine block, it is not your magneto. 
  • Connect the plug wire to a spark plug and ground the base of it against the engine block to see if the plug will spark.  
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The plugs spark when withdrawn from the engine. They also trigger my timing light, which is how I set the timing. When I insert the plugs, the timing light no longer is activated. Hence, I think the plugs are not sparking.

 

Phil

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I had that happen some years ago with a flat head Studebaker.

Champion plugs would not fire under compression no matter how far I towed it.

Plenty of spark at the plugs when they were laying on the head.

A set of Autolites (I think) fired right up with the starter.

 

But could it be that your with your mag, maybe with compression it doesn't crank fast enough to make spark?

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With all the plugs in the engine take another plug and connect it to the plug wire leaving the extra plug external and crank the engine. Does it spark? If not the mag does not make enough voltage with the slower engine cranking speed to fire the plug. Check the cranking speed with the plugs out and with the plugs in. The engine should turn over at least 125 to 150 RPM's at cranking for the unit to start well. If the cranking speed with no compression is 100 and with the plugs in the compression drops the speed to 70 I would look at why the RPM's are so low and see if it can be improved. The other cause would be if the cranking speed is normal and there is no spark from the Mag then it is not producing enough voltage to jump the gap. Connect a 12 volt test lamp to the plug wire on one end and ground the other end of the test light.With the plugs in the cylinder, crank the engine. see if it makes enough voltage to make the test light flash. If so the mag cannot produce enough voltage to fire the plug at that cranking speed. If cranking speed is normal the mag is weak. If cranking speed is low see why and get it to where it needs to be.

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Thanks. I'll give that a try. The magneto was gone over by a reputable shop about ten days ago and I was told the spark was good. I'm turning the starter motor using jumper cables, but not sure of the rpm. Being 6-volt, it is not very fast.

 

Phil

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Get rid of the jumper cables and use the battery cables that you intend to use for your final install.

For testing you need to replicate what will be the finished product.

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Flywheel RPM is about 47. I'm going to run some 2/0 battery cable with correct connectors, like it will be in the car, and see if that makes a difference.

 

Phil

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Hi to MochetVelo,  my anecdote was with a BSA motorcycle twin with a Lucas magneto. It would faintly spark at the plug when outside but it didn’t have enough to ‘bridge’ under compression. We had the magneto rewound at great expense,and when returned, it would spark at least 12.7 mm to you and I, or  1/2 inch to everybody else. The bike started first kick; .....maybe the second kick! The difference was unmistakable. The magneto rewinder man explained that is typical of old magnetos in that you cannot produce enough voltage at low speed. When someone is testing by spinning it by hand you would be surprised how  fast you can spin it. Certainly a lot more than 47 rpm. It may seem o.k. .......  moving on . To overcome the problem of slow cranking speed,  later in the 1920s someone invented the ‘impulse’ magneto. This was an attachment to the drive end which , simply explained, was spring loaded. When you cranked the motor over at (47 rpm!) the magneto would stop turning because of the magnetic resistance. As the motor approached TDC ,the spring in the impulse would be stretched to its limit and would ‘trip’ the magneto . This would spin it over at great speed. When working correctly, no matter how slow you turn the motor the ‘impulse magneto’ would spark. A great invention, unfortunately motor cars had moved on to distributor and coil ignition by then. Later they were used a lot on stationary motors. In the old days, my father would often remove the magneto on cars and, with a basic adapter, fit a distributor and coil. For him, rewinding a magneto was very costly. Viva Citroen. 

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The problem was the cranking speed. As Jack suggested, I swapped out the jumper cables for 2/0 battery cable on the + side and the starter motor turned much faster. Anyway, it now produces a spark, according to my timing light.

 

Phil

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