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Old 08-30-2010, 04:47 PM   #16
OldPete
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Quote:
Originally Posted by Mista Vern
balast resistor
I say BINGO!

Reversed polarity or a failed condenser will not cause over-heating.
No condenser will cause(edit: points to burn up from arcing) rough running, back-fire and no high rpm operation.
Power will be weak but the coil will not over-heat.

Automotive coils of old were oil filled and should be mounted in a comparativly cool place, vertical with the secondary up.

I had to zip-tie a NAPA coil to the Goose to get home once(700 miles).

OldPete screwed with this post 08-30-2010 at 05:32 PM
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Old 08-31-2010, 02:04 AM   #17
motu
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Electronic igns have current control by module or computer,and usually have less resistance.Points coils just have to take however much current the points feeds them,and have higher resistance.You need to check primary resistance on those coils,but seeing as they are both now buggered,not much point.Some early electronic igns used a ballast resistor....you need to find one.

Luckily you are in a place where you might just find one on the side of the road.Have a look through a wreckers yard - you are looking for a white ceramic resistor next to the coil.Also look for a waste spark coil pack,plenty of cars have them,talking Japanese here.A 4 cyl will have 2 coils on a bracket - that gives you two twin output coils,hopefully very much cheaper than a HD coil.
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Old 08-31-2010, 02:14 AM   #18
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The coil is overheating because you are pulling to much juice through it's primary.

The circuit is strait power from the ignition to one side of the coil primary. The other side of the coil primary goes to the points and then to ground. So when the ignition is on and the points are closed, juice flows through the coil to ground.

When the points open they interrupt this flow on the ground side causing the high voltage spike in the coil secondary windings that is then routed to the plugs to make sparks. The condensor is parallel to the points just to suppress arcing when the points open.

If you sit with the ignition on, engine not running and points closed, there will be a constant flow of juice through the coil primary, which can overheat it. Ditto if the points are shorted to ground---but then it will run like major crap if at all.

The electronic ignition can switch a lot more power than points so you use a coil that can take more power. It will have a lower primary resistance to allow the flow. maybe 1.5 Ohms for a single dual nose coil firing two plugs on two cylinders.

The points setup can switch less juice so think 3 ohms for a single dual nose coil firing two plugs on two cylinders.

A pair of single nose coils in series firing two plugs on two cylinders will have the combined resistance of the coils. So two .75 ohm coils in series gives a 1.5 ohm circuit. But put them in parallel and the resistance halves. Two 3 ohm coils in parallel gives a 1.5 ohm circuit.

If the resistance is too low, there will be too much current and you might cook the coil (and the points). If there is an external resistor and it is bad and not resisting enough, this could do it.

As something heats up, it's resistance increases. Just to keep in mind.

Pull the hot feed to the coil(s). With the ignition off and the points open, measure the resistance from the hot input terminal of the coil to the "hot" (wired) side of the points. If the resistance is too low (I'd guess under 3 ohms) figure out why.
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Old 08-31-2010, 04:21 AM   #19
AntonLargiader
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I didn't see anyone mention dwell so I will. Points saturate the coil for a certain (fixed) percentage of the crank's rotation. The spark happens when they open, then they close again and charge the coil. Because of the inductive nature of a coil, coil current increases as this 'dwell time' gets longer, so if you have the points closed for too long you will heat the coil exponentially/logarithmically/biorithmically/something..

Points setting specs are basically dwell (often set by the gap) and the timing itself. They go hand in hand. It's possible to get the right timing with too small a gap, meaning you get too much dwell. On a slow-running motor, with lots of coil soak, that could make a difference.
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Old 08-31-2010, 05:42 AM   #20
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Dwell-time can also be increased by mechanichal failure at the points (the part that touches the cam) and the cam itself.
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Old 08-31-2010, 05:52 AM   #21
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I had a similer problem on my 59 panhead many years ago. It was frying points within just a few minuets of installing them. Turned out it was a bad wire. As has been mentioned, when the coils are drawing too much amperage through them they will get hot. In my case the wire going to the points was broken inside the insulation (you couldn't see it) and in order to push the voltage across the bad area, the current went way up.

I would check the wire on the positive side of the coils and see if it is nearly broken some where. If you have a bad ground (as has also been mentioned) that could cause it also, but I would expect it to be frying light bulbs too.
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Old 08-31-2010, 06:02 AM   #22
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Does it have a resistor in line with the coils? old school car coils use a low value resistor ( About 1 Ohm ) to lower 12 volts down to about 9 volts to the coil. I had this problem years ago on an old truck because someone bypassed the resistor. The coil would get so hot it would start missing then
work agaain after it cooled.
If you have a meter check how many volts are at the coil.

Hope this helps. D


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Old 08-31-2010, 07:01 AM   #23
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Quote:
Originally Posted by Furious D
Does it have a resistor in line with the coils? old school car coils use a low value resistor ( About 1 Ohm ) to lower 12 volts down to about 9 volts to the coil. I had this problem years ago on an old truck because someone bypassed the resistor. The coil would get so hot it would start missing then
work agaain after it cooled.
If you have a meter check how many volts are at the coil.

Hope this helps. D


This happened to my 85 Range Rover......small engine fire lead to some burnt wires that I replaced but the power wire to the coil was listed in the diagram as a "resistor wire". I tried in-line resistors of all types but had a hard time finally getting 9 volts going to the coil. And yes for a while there I had a hot coil although it never failed me.
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Old 08-31-2010, 06:58 PM   #24
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Any HD running points or an intermediate 79-80 electronic ignition should be have 5 ohms total resistance in the coil. Most elec are 3 ohm and will work but poorly. Part number 31609-65A should be the ticket if you can get it OEM. I have officially sworn off anything but factory Harley electrical products. Quality and very reasonable. Hope this helps. Craig
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Old 09-01-2010, 09:07 AM   #25
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Quote:
Originally Posted by rtwdoug
I was riding when it boiled out & died, had been ridin about 90 minutes straight.

I'm clueless, the odds of a new HD oem coil being bad are slim.
specially to work fine for 4 hours of riding
the 1st coil lasted about 10k miles
Not unknown for a new part to fail. If the insulation in the coil primary fails and it bypasses some of the windings, the resistance will fall and it will pull more amps and run hot. Most points can handle about 3 amps, 13v at the coils and you need about 4-5 ohms to keep everything working.
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Old 09-01-2010, 09:12 AM   #26
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If there's a Harley shop there, then there oughta be an auto parts store there, and there would be your source for an automotive points type coil. Also, when you went to the Harley shop, I'm certain they sold you something that was for an eight year old Harley at the very oldest, because they only carry stuff back that far...I think there's a good deal of suspicion here that you should be using a points type coil for your points ignition, makes sense, if you think about it, so take care of your problem thusly, and I'll bet you won't have any more problems...
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Old 09-01-2010, 10:12 AM   #27
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Just saw this thread. You're getting some good info here, Doug.

Hot wire continuity.

Good ground.

Correct dwell.

Correct coil resistance.


It's gotta be one of those or a combination of a couple of them.




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Old 09-01-2010, 06:23 PM   #28
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Timing...

Maybe put a dwell meter on it. Excessive dwell will cause the coils to overheat. This seems unlikely on a twin, but is possilbe and is easy to check. Most dwell meters only go down to a 4 cylinder setting, so double the expected dwell spec for your bike. This ASSUMES a singe set of points. Anything from 70 to 150 degrees should be fine.
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Old 09-01-2010, 09:21 PM   #29
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hey guys
thanks for the tips & ideas.
the car coils are hot, but working
I got a set of russian Izh coils as a backup, as spare parts aint everywhere here.
when I get somewhere more civilized, I'll look into it better

Doug
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Old 02-20-2011, 05:13 PM   #30
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Did you ever find out what was the cause of this issue Doug?
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