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Posted by D Sherman [24.32.202.83] on Tuesday, May 18, 2010 at 18:32:07 :
In Reply to: problem starting my 1947 power wagon posted by mike fagan [69.122.243.150] on Tuesday, May 18, 2010 at 16:12:56 :
When cranking, the voltage is dropping too low to make a spark. When you let off the starter, the voltage jumps up enough to make a spark and if the engine is still spinning from inertia, it will start at that point. Happens a lot with regular starters (with solenoids) and 12 volt systems too, as the battery gets weak or there's a bad connection at a battery post. Anything else that contributes to a weak spark can be part of the problem here.
You can use a timing light, clamped onto one of the plug wires or the coil wire, to see if the spark is sparking while you have someone crank the engine. If the problem is as I described, the light won't flash while they're cranking but will flash a couple times after they let off the starter pedal. Then you have to figure out the cause of the weak spark -- weak coil (or wrong voltage of a coil), bad ballast resistor or a ballast resistor installed when there shouldn't be one, points dirty or mis-adjusted, bad connections (especially the main battery cable clamps and ground cables), etc.
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