Author Topic: Good Coils Gone Bad?  (Read 2764 times)

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Offline mower_mulisha

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Good Coils Gone Bad?
« on: July 08, 2012, 03:27:44 PM »
Is it possible for a Briggs coil to work great then one day not funtction after 5500 or so RPMS. I know I once purchased some cheaper "Aftermarket" coils and they only worked on low RPMS..once they got to around 5000-5500 the motor sounded like it had a Rev Limiter on it. Coils just didn't work at that point. I switched to some Briggs ones I had laying around and the motor turned over 7000 no problem. So my question is can a Briggs coil eventually do the same thing...(Start to lose function above 5500 RPMS) thoughts were always either a coil works or it doesn' is this possible or have you had it happen??

Offline birdman_express

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Re: Good Coils Gone Bad?
« Reply #1 on: July 08, 2012, 08:13:41 PM »
Coils can be some the most cantankerous things.
I have had a few that would idle well, but give it some
gas and they would die. Put a new coil on, and run it through the roof.
Sometimes the problems would not show up, til they were up to
operating temperature, then show the problems you describe.
Other times they would die, til they cooled off, then you could run
them til they heated up again. I have noticed, for the most part,
that if a coil does not just die without warning, the upper RPM's
will be where you notice the degradation first. Hope that helps.

Marc Baker

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Offline redline

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Re: Good Coils Gone Bad?
« Reply #2 on: July 08, 2012, 08:24:54 PM »
Darrin, another thought I had after you left Fifield,  could the coil wires or plug boots be arcing to the tins/block? I see this happen quite a bit in the automotive world. Usually it leaves carbon tracking (thin black line where voltage was arcing, and if it's on the porcelin of the spark plug, cannot be wiped off), but Briggs coils are nowhere near putting out the voltage of the auto ignitions, so it may not really show up. I have witnessed coils arcing to themselves, arcing in the center of the wires, and also where the plug boots are.
Ed King

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Offline Honey Badger

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Re: Good Coils Gone Bad?
« Reply #3 on: July 09, 2012, 07:14:20 AM »
As a coil heats up the insulation around the windings do as well.   After this stuff breaks down the windings short to themselves changing the resistance of the winding.  This new resistance may be sufficient to fire off at low demand throttle angles and duty cycles or rpm.  Once you up your cylinder pressures or cross the coils new rpm threshold it cannot keep up ans you get your misfire. 
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