Wildguzzi.com
General Category => General Discussion => Topic started by: Turin on December 02, 2025, 10:52:08 PM
-
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.
(https://i.ibb.co/WXv4b5h/se-frame-exhaust-1.jpg) (https://ibb.co/WXv4b5h)
-
With the plug wire off, the energy will find the next path to ground is my first thought.
-
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.
-
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.
-
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.
(https://i.ibb.co/v6Rqbzg1/mz00-r.jpg) (https://ibb.co/v6Rqbzg1)
Two ground points aimed at HV terminals visible in this photo.
-
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
-
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
-
Using a timing light to check your full advance while running? Sounds like your running issue
-
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?
-
no plug when the coil arced, gave me a scare. won't do that again.
Engine pulls like a freight train until 6500 RPM, then acts like it's hitting a rev limiter. Fluttering. I've tried every main jet from a 155 to a 225. needles have been raised, lowered etc.
Good battery, Charging system is working. Timing and advance checked. Valve gap checked.
Dyna III ignition (I've tried two, no change)
Dyna 3 ohm coils ( I'll try swapping these for another set)
NGK bP6ES plugs
NGK 8 mm copper core wires
It must be something stupid.
-
What is the chance that something is floating at 6,500? Maybe a valve spring is broken and the valve doesn't fully close at the higher RPM?
Or your advance is not fully advancing?
-
About the only things left are cam timing or valve float. I can think of a couple others but the symptoms would be slightly different.
-
Is it getting enough fuel? Are the filters on the fuel taps clear of rust etc?
-
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.
(https://i.ibb.co/v6Rqbzg1/mz00-r.jpg) (https://ibb.co/v6Rqbzg1)
Two ground points aimed at HV terminals visible in this photo.
Thanks for the picture. I like that design. The good ole days.
My father had a different method for grounding out/isolating a single spark plug.
He would make one of his sons hang on to the spark plug in question.
If you flinched, he knew that spark plug was getting juice. Then he feign ignorance and ask "What the Hell is wrong with you?"
Some may think that cruel, personally, I think having your brother or mother-in-law hold the plug sounds more humane.
-
Clear fuel line and plenty of fuel flow. Valve springs are not broken.
-
ran into that problem with my modded LM 3, air filter was the restriction.
-
How do you know it's getting enough fuel?
Yes, DO check for obstructions in the air flow.
It's hard for me to believe there's anything mechanically wrong with the engine because it runs great up to 6500.
I've heard bad things about the NGKs, so I'd try Denso plugs.
When you 'hit the rev limiter' what happens giving it more throttle? Does it try to go faster or stall out. Then if you back off the throttle, does the engine speed up? What exactly happens at 6500?
Fuel, air and spark are needed to run.
Once it's determined they're all there, is there enough fuel, air and spark?
After that, you're getting into fine points of timing and oddities.
Does the engine rev to redline while stationary?
-
ran into that problem with my modded LM 3, air filter was the restriction.
This was a head scratcher until I found it. Bike ran great except at wide open. Turns out that the closed end of the filter needs to be at a minimum two times the diameter of the throat . Theoretically it should be 2.5. It usually isn’t a problem until we do some tweaking . It’s measured from the carb opening to the end of the filter.
-
Let's see if I read you correctly, John: For a 40mm Dellorto, the distance from the entrance into the carburetor to the closed end of the filter should be 100mm or more? Minimum 80mm?
Intake, including filter dimensions, and exhaust sizes have an effect on an engine's performance.
I worked at an auto repair shop for a while back in the late 80s and there was a VW bug that had us pulling out our hair. It wouldn't run past mid range. We'd gone through everything. Finally the boss decided to replace the fairly new exhaust. Didn't make sense, but we'd gone through everything else. As it turned out, the owner had driven through a deep puddle which had shocked the hot muffler and dislodged a baffle. Ran fine after that!
-
2 sets of plugs tried. I'll pull the K&N filters and install the velocity stack/bell mouths. cant hurt.
-
Turin, what type of spark plug wire ends are you using?
Kip
-
Let's see if I read you correctly, John: For a 40mm Dellorto, the distance from the entrance into the carburetor to the closed end of the filter should be 100mm or more? Minimum 80mm?
Yep, that’s right. I got the information from this book https://www.ebay.com/itm/177555593782
They did a lot of testing on a 500cc one cylinder so it is relevant to a Guzzi. Another thing I remember from it is that the exhaust box, or muffler should be ten times the volume of the cylinders, mostly only possible on a stationary engine.
-
Kip,
NGK Red wires. https://power-barn.com/ngk-racing-cable-spark-plug-wire-90-degree-boot-cr2/ (https://power-barn.com/ngk-racing-cable-spark-plug-wire-90-degree-boot-cr2/)
(https://i.ibb.co/WXv4b5h/se-frame-exhaust-1.jpg) (https://ibb.co/WXv4b5h)
-
Kip,
NGK Red wires. https://power-barn.com/ngk-racing-cable-spark-plug-wire-90-degree-boot-cr2/ (https://power-barn.com/ngk-racing-cable-spark-plug-wire-90-degree-boot-cr2/)
(https://i.ibb.co/WXv4b5h/se-frame-exhaust-1.jpg) (https://ibb.co/WXv4b5h)
THAT IS SEXY!!!
I love the mafia sticker!!