Arcing Dyna Coils

[Turin]

Anyone else experience this issue? I unplugged a wire from the plug and saw a bright arc from the coil to the frame when I cranked it over.  These are the 3ohm (new old stock) and they are arcing from the coil body. no cracks in the coil body.

Backstory - I'm chasing down a running issue on a modded lemans 1000 engine. the power curve hits a wall at 6500 rpm. Down low it's great. I've tried different main jets, checked the timing and advance, valve lash, changed out the dyna ignition. Hoping that a coil swap will do the trick.

Not a mechanic... more of a glorified parts swapper.

[John A]

With the plug wire off, the energy will find the next path to ground is my first thought.

[nc43bsa]

With the plug wire off, the energy will find the next path to ground is my first thought.

Indeed.

GM HEI coils have been known to do similar things.  The coil has high enough voltage to spark, even if there is no spark plug connected.

[SIR REAL ED]

With the plug wire off, the energy will find the next path to ground is my first thought.

Correct.

I have long heard that you should always ground the plug wire when turning the engine over.

It might be a old wife's tale, but I have heard that some ignition systems will damage themselves if you don't give the energy a path to dissipate.

[n3303j]

Yup, when you pull a plug wire on a running engine the energy has to go somewhere. When it arcs across the insulating material it turns a bit of it into carbon. This makes the path easier to follow each subsequent time. Eventually you end up burning an alternate path into the insulating material.

Safe way to isolate cylinders for diagnosis is putting a bare metal extender on the plug top and connecting the plug wire to the top of the extender. When you want to disable a cylinder you touch the head with an INSULATED handle screwdriver and bring the shaft to rest on the plug extender. Spark shorts to case ground and energy sagely dissipated. I always made plug extenders with the plug screw on top terminal with a long metric screw (head removed) threaded halfway through it. That screws easily on top of plug and plug wire fits the metric screw.

Early sparking systems (R60/2, Ural) had two grounded and pointed tabs that were gapped near the coil outputd so that if a plug wire dropped off the spark would jump from the metal terminal to the point on the grounding tab. Insulation was not so good in the '50s.



Two ground points aimed at HV terminals visible in this photo.

[michaell32]

I always used one of these if I needed a plug out while cranking. You can use an alligator clip to hold it to the engine or frame.  https://www.autozone.com/test-scan-and-specialty-tools/ignition-tester/p/duralast-in-line-ignition-tester/1267200_0_0?searchText=Spark+plug+tester

[Moparnut72]

Always ground an unused plug lead while cranking or running an engine. A very likely way to destroy a coil. The energy will do whatever it has to do to find its way to ground quite often destroying anything in its way.
kk

[chuck peterson]

Using a timing light to check your full advance while running? Sounds like your running issue

[wirespokes]

So now the question is - does the coil arc to ground with the plug wire replaced?

When everything looks good, yet there's still a problem, that's the time to look deeper. Looking deeper requires knowing more about the systems being checked.

Your high speed running issues can be electrical or fuel. A little more data might narrow the search. Give us more data what happens. Does the engine gradually run out of steam? Give it more throttle and it bogs? Backfire? Bucking?

What bike is this? Points? Electronic ignition?