Hot Coil

Hello Access Norton members and Happy Veterans Day to everyone!
I have on my 72 750 IS, one Original coil still. It was one of two which were on the bike when I found it, dated 72. One stated to leak after a year or two, and I replaced it with a 1974 coil. Today after a short 6 mile run, I came back, and noticed the idle was off, a little high earlier in the ride, then a little low at home. I fettled the idle screw while on the ride, thinking it was the new battery giving full output. I did check the battery with a meter when I put it in, and the readings were good at idle, and at 2500 rpm. (12.8 to 13.2 at idle, 13.7 at 2500).
Just to be sure, I was looking over the wires, and noticed the coil wires had dropped down, onto the head. No insulation burned , no bare wires thankfully. But, the left original 72 coil was HOT as a pistol! The other one, was fine, normal warm to the touch.
I am assuming it is just time to replace the original 1972 coil. I have new Lucas ones in the box in my gagage and will swap out both old coils.
I will also lift off the gas tank when I do this, to be able to inspect the harness, and make sure the coil wires are tie-wrapped up out of the way and not in danger of touching the head again.
Any observations on my HOT vintage '72 coil? Any thing I need to look into? I'm assuming the original 72 coil was just about to give up the ghost, and I happened to catch it in time.

Just a note, I restored the bike 10 years ago, and it has been a trouble free machine the entire time. I've clocked about 7k miles since then, no issues other than worn tires and brakes.
Thanks, Rob

P.S., that is the bike in my avatar

What type of ignition are you utilizing? If stock, maybe a capacitor may be the culprit.

On points with the engine not running one coil is permanently on and gets hot, left long enough and it will destroy it. If it's on EI or you turned off a running engine then the hot coil is toast.

kommando said:

On points with the engine not running one coil is permanently on and gets hot, left long enough and it will destroy it. If it's on EI or you turned off a running engine then the hot coil is toast.

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Deckard: Ignition: All stock with modern equivalant parts: replaced original rectifier with a solid state unit, replaced original assimulator with solid state unit in the can, but ignition is otherwise all stock. I have not replaced the condensors since the restoration. Its time for that.

Kommando: I did screw up this morning and when I returned, I was distracted by the idle. I did a little idle adjustment, then turned off the bike at the kill switch on the cluster, and completely unlike me, LEFT THE KEY ON! It was on for a long time, while I was fussing with the side cover, and did not see it for many minutes. I then removed the fuse as I always do after I'm done riding, and when I went to pull the key, it was in start 'on' position! That makes total sense!
Would you say then, it's time to replace the coils? Or will that old 72 coil live to ride another day?

Does leaving the key on fry the points as well?
I did that once on a car I had and the points were carbonized and the battery was dead.

It's a perfect day to retire that coil I would say, and salute it as a veteran of 52 years service. Not bad eh? poor old Joe Lucas get's a pretty bad rap in my opinion.

One thing I did was to remove the Lucar blade terminals on the coils and loom and replace them with hoop terminals. (one more potential loose connection point gone).

cliffa said:

It's a perfect day to retire that coil I would say, and salute it as a veteran of 52 years service. Not bad eh? poor old Joe Lucas get's a pretty bad rap in my opinion.

One thing I did was to remove the Lucar blade terminals on the coils and loom and replace them with hoop terminals. (one more potential loose connection point gone).

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Yup retire the coils , they did their service . There are modern upgraded ones , same size and fit . Not expensive . Do not crush them with the clamps . 2 lb. of clamping force is so little . I replaced blade with ring type too . Enjoy .

Even if it still works its a failure waiting to happen. Change both coils, do the clamping bolts up only enough to hold the new coils, the clamps distort the body and cause internal shorts.

Look at the manual - the torque to clamp the coils is some ridiculously low amount - 0.8 ft lb. (10 inch lb) from memory.
With this very low torque use Loctite Blue (243) - over-tightening will kill the coils.
I'm sure I have killed coils with careless, ordinary spanner work in the past!
Cheers

I'm reading this with a tear in my eye... I'll enshrine those coils in a little box, on the wall, and light a candle...
Yeah, I say same about Lucas-Everyone talks about how awful their components are, yet, 52 years of service. I always felt the bad rap came from, well, a few places, but for this thread, the collapse of Norton in the 1970's meant as these bikes rode on, they never benefited from the continual advancements achieved at Honda, Yamaha, Kawasaki, etc. Norton Motorcycles became stuck, or frozen in time until the small company electronic advancements came along.
Coil: Yes, just a little cinch, I apply a touch of Loctite as well. First time I saw a creased coil on my buddy's 74 some 40 years ago was an introduction to the 'charms' of a Commando.

I'll pass on a strange tip a friend of mine uses on vintage cars and bikes with spade connectors. He keeps his machines stock when possible. After a good cleaning, he applies a pin head touch of super glue. Sounds crazy, but it's just enough to hold the spade on. But not enough to prevent removing it when it needs to be worked on.
I myself do not do that, just passing it along.
Ring connectors sound smart.
Again gentlemen, Thank you all for the sage advice. Once my bride clears me for garage duty today...

heres a very good, detailed explanation of how coils work

Thanks Jimbo! And Happy New Year!
I have a friend (yes, just one) that is always trying to teach me how to use my multi-meter... He's one of those engineering aircraft types. I listen, then fumble along...
BTW, thanks for everything, the coil issue on the 72 is resolved, and she is running fine!
I'm going to start a new thread, with a new mystery: Clutch. First time one has perplexed me to this point.
Rob

You can check the coils primary/secondary resistance as per the workshop manual to see if they are done. My '74 850 coils still doing a fine job...put 20k miles on them past 7 yrs alone.