MochetVelo

Magneto: No Spark

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Posted (edited)

Trouble starting my 1924 Citroen engine. It uses a Bosch ZA4 magneto (rebuilt about 10 years back) which produces a nice blue spark in the plugs when they are out of the engine. When screwed in the head, however, the plugs make no spark. At least I assume so: My timing light (seen in photo clamped to plug wire #1) illuminates correctly when plug #1 is unscrewed from the engine and sparks, but it does not light up when the plug is screwed into the engine. It's a 6v system, so the starter turns rather slowly (maybe 80rpm), but I think it should be fast enough to start. Points and contacts cleaned, and wires tested for continuity. Any suggestions?

 

Phil

 

 

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Edited by MochetVelo (see edit history)

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With the plugs in the engine connect one plug outside like you have been testing it to confirm no spark, If there is spark present I would open the gap of an old spark plug to 3/16" and see if the spark will jump that gap. If it does not jump 3/16" it probably will not fire a cylinder under compression. if it doesn't jump 3/16" double check the points to see if they are shiny clean the whole face of each point. If they look good I would replace the condenser if it has one and retest.

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Thanks. The plugs all light when loosened, but not when installed. I'll try the 3/16" gap test. I'm thinking the magneto is weak even though it was rebuilt with new condenser..

 

Phil

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It appears that magnetio does not have in impulse coupling which means the engine must turn over much faster to develop as hot a spark as if there was an impulse.

I would narrow the plug gaps to .020" and see what happens.

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I wonder if hand-cranking might get it going. I'd need to install it in the chassis or rig up something to do that. I also want to wire up the battery better, as I'm just using jumper cables.

 

Phil

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You are probably loosing amperage through the jumper cables.

Can you rig up real cables? heavier the better.

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I worked on farm equipment for my last real service job from 2013 thru 2016. The condensers we get now are made in area's that there are no or very poor quality control checks. I have seen several condensers during that time which work for the half but will not generate the field collapse and fire the plug . These were on older equipment which had complete tune ups that went to the field and didn't work more than a hand full or two of times and then they would begin to start hard, no start or run for a short time and shut off then not restart. I would definitely try another condenser if it were me. I know a great starting engine will fire a plug with 3/16" gap when external of a cylinder. If you have a plug tester available that adds simulated compression pressure (using air pressure) to the end of the exposed plug while firing voltage to it you will see the more pressure the higher kv the plug needs to fire correctly. Blue spark is great but if it can only jump .03" when external of a cylinder with no compression pressures when you add the compression and fuel mix it will fall on its face. The other thing I have noticed is the spark will arc to ground when compression is on the plug if external arcing is the path of least resistance. I do not think this is your issue because the timing light does not trigger. You can check for this by trying to start in a dark area with no lighting. 

I do not know it all but I will try to help.

Best wishes on your repair 

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