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Old 08-29-2019, 10:48 PM   #1
DannL
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Default Don't discount the coil as being bad . . .

I did for several months . . . and now wish I had replaced it on day one. The engine always sounded rough as if it was firing only on some cylinders, randomly. Even after replacing the carburetor, plugs, and points, and numerous timing alignments. Replaced the coil and now it sounds like a different engine. Definitely firing on all 4. Plus as a bonus, I was able to turn in the idle screw another 1/2 turn and see the engine idle at a nice low rpm for the first time, with no coughing and sputtering.
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Old 08-30-2019, 12:34 AM   #2
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Default Re: Don't discount the coil as being bad . . .

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Old 08-30-2019, 05:36 AM   #3
Patrick L.
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Default Re: Don't discount the coil as being bad . . .

It happens. Anything can fail. Was the coil oil filled ?
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Old 08-30-2019, 07:08 AM   #4
BRENT in 10-uh-C
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Default Re: Don't discount the coil as being bad . . .

I actually have a vintage Coil Tester, and I find many coils on Model-As are marginal at best. Below is a few pictures that I will use to illustrate something.


Image $1 shows the unit connected to an original coil that is typical. If you look closely, you will see the pointer is on a eccentric that allows the gap to be widened as the rotator is turned. Notice the flame it is putting out.


Image #2 shows the spark the same spark with the lights off for better spark clarity. You will see the spark is very fat & blue.


Image #3 shows the scale. This particular coil started to have erratic spark (missed three or four times and then would be solid for 10-15 before it started braking down again) at a gap not much greater than what you are seeing in the picture. Note the rating on the scale as Poor.


Image #4 shows that the spark gap is about 3/16" wide. When the gap was increased to about 1/4", the spark became erratic, ...and would only jump intermittently above that.




Now what most people do not realize is the distributor rotor gap is adjustable, and the wider the gap of the spark must jump to seek an earth ground (-add the rotor to body gap distance along with the spark plug gap distance together) the harder it is on the coil.


Then add into the equation how the spark must jump thru pressurized air inside the combustion chamber, and you quickly find even a coil that can jump a 1/4 inch gap is marginal at best. A marginal output of a coil equals a poorly running engine from my experiences.

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Old 08-30-2019, 10:31 AM   #5
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Default Re: Don't discount the coil as being bad . . .

Not on a Model A, but I did a major tune-up on my '42 Jeep - plugs, points, wires, distributor cap, rotor, etc. - everything except the coil. So I'm at an event and the thing won't run right, get it home and pull the coil (not an easy thing to do on a Jeep) and its substandard according to Les's book (yes, the Model A book which I find is pretty generally applicable) so I replaced it and voila, no more problems. Should have known from my Model A's - first things I suspect when an A runs rough are the coil and condenser...
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Old 08-30-2019, 10:32 AM   #6
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Default Re: Don't discount the coil as being bad . . .

Brent, what does your tester use to simulate points closing and opening? A vibrator of some sort?
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Old 08-30-2019, 11:42 AM   #7
DannL
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Default Re: Don't discount the coil as being bad . . .

Quote:
Originally Posted by Patrick L. View Post
It happens. Anything can fail. Was the coil oil filled ?
This coil appears to be original if not just ancient. Has a brown bakelite top with Standard on it. Feels solid with no sloshing inside.
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Old 08-30-2019, 11:48 AM   #8
BRENT in 10-uh-C
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Default Re: Don't discount the coil as being bad . . .

Quote:
Originally Posted by katy View Post
Brent, what does your tester use to simulate points closing and opening? A vibrator of some sort?


Yes, I think so.


It also has a feature that I do not know how it works, but there is a heat selection where supposedly it will heat a condenser or a coil to cause it to break down. Since this unit is DC battery powered, I can only assume it is creating high voltage thru a transformer to cause the item to get hot.
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