Had a coil failure, first real problem on my N15, but it seems resolved with replacement.

Went for a ride, me on Laverda 3C, son on N15. 15 mins in, the Norton sputtered and died. Got it running after a rest, checked fuel flow as best we could at the clear filter. Switched bikes, nursed it home, a bit of a pain; coasting and bump starting when I had to did the trick. I have had issues with bad fuel, but this seemed to be either fuel flow or coal heat soak. The bike has 2K miles on a Wassell Vape mini and dual-lead Tri-Spark 12V coil, and it has been flawless until now. I checked the flow & cleaned the carb. I recently put on a cutoff for sumping, so I checked its operation & wiring, and also the ignition switch, which was a bit suspect - I found a crack in the plastic on the back, and epoxy-welded it, and put on new crimps for the terminals. I also drained and replaced the fuel.

I tested the coil, but it's hard (for me anyway) to replicate heat soak on the bench. Ohms reading OK on primary, a bit higher than spec on secondary. I didn't want to throw a bunch of $ on a suspected problem, so I got a $30 3-ohm 'Magna' coil, which is a copy of a Dyna. All is good now, running beautifully. Maybe mounting the coil under the fuel tank overheated it? It's insulated as best I can from vibration with rubber, and never gave problems before. I moved the new coil an inch forward to hopefully cool it better. We'll see, but I'm happy that it's running again.

I've had several dozen bikes and cars, and the only other coil problem I had was a free Suzuki GS1100E that had some junk Chinese coils - it had the originals in the parts pile that came with it, and swapping fixed it right away. Seems that on my vehicles at least, coils live in hot places and survive just fine. Perhaps it's nothing to do with external heat? I can understand vibration, but my N15 isn't bad at all in that regard. Even considering that it's a Norton, it's sort of amazingly smooth the way it's set up. C'est la vie.

I had a Dyna 3.0 ohm twin coil that measured 2.9 ohm, the boyer overheated on hot days and needed 10 mins to cool down so I could get another few miles. I then went for a cheap Harley one with 3.4 ohm and no issues since.

A failing dual coil will send me on a "guess then test" shotgun troubleshooting binge.

I use a not cheap 3.4 ohm Compu-fire Harley style dual firing coil on the P11 now, but it was probably cheap for the company I bought it from since it was made in China pre-Trump tariffs. The Harley style dual firing coils do work well.

I had a dual plugged '79 BMW R100RS that had a pair of Dyna coils on it. Never a hiccup. I should have kept that one. It was a nice ride and would have been a perfect vintage touring bike in my golden years. but I digress

The Wassell instructions specify a 3.0-ohm absolute minimum, which the new one measured exactly. My son's CB175 project had a no-name HD coil zip-tied to it and also a TEK MP-08 under the tank. Maybe heat soaking too, thus the replacement. Those are considered a good coil, original on Goldwings and a common choice for Japanese 4's per KZrider and GSresources forums. The HD coil is too big for my liking, but I suppose I could have zip-tied it to the frame for a test. The TEK measured out at 2.5, so it was a no-go.

For others that have yet to experience a failing coil... First thing that may come to mind in the guessing department is a carburetion problem. It feels like running out of fuel while failing and is annoying as heck getting back home. I've had it happen with HEI coils for GM cars as well and wasted a lot of time troubleshooting (AKA testing) the wrong things before replacing the coil. Such a great feeling when everything comes back to life after making a simple parts change.

Schwany said:

For others that have yet to experience a failing coil... First thing that may come to mind in the guessing department is a carburetion problem. It feels like running out of fuel while failing and is annoying as heck getting back home. I've had it happen with HEI coils for GM cars as well and wasted a lot of time troubleshooting (AKA testing) the wrong things before replacing the coil. Such a great feeling when everything comes back to life after making a simple parts change.

Click to expand...

Indeed. I had a condenser fail on my first car, a 1972 Toyota Corona. What a ***box it was, even being in as-new condition with only 60K miles when purchased. It was an absolute dud to drive, but that 18RC 4-banger was pretty tough. I somehow figured out that the condenser was bad, despite my absolute ignorance. Replacing that little cheap part to make it run again was nice. Now I have a 'check shifter' warning on my '14 Grand Cherokee, which has been a great car for nearly 100K miles since I got it nearly new (best $20K purchase ever), and I'm terrified.

Several rides, and it's as sweet a ride as ever. Hopefully, this coil lasts longer than 2K miles. I'm happy with how the bike turned out, it is a really fun, easygoing and smooth (??? a Norton ???) ride. The main elements are very nicely sorted - engine, carburetion, clutch/gearbox, suspension. The only things bugging me are the speedo. tach, and fuel cap. Both gauges are rebuilt. The speedo needed a new cable as it had one from a Commando that was about 4" too long. When I first installed the new proper Andover cable, it was perfect. But now it has started to jump around in the 40MPH range, then smooths out again at 55 or so. The gearbox at the rear wheel is NOS, so with a new cable and a properly rebuilt speedo, all was perfect - for about a week. Then it got jumpy. The tach is jumpy around idle, then smooths out. That cable is fine, lubed, and not taking any funny bends. The tach drive gearbox is clean and greased. Sheesh.

Also, the bayonet-style fuel cap is terrible. I have to gently cinch it up, and if I turn too much force, it goes over the hump. Probably a gasket, but I put a new one on. Of all the things to be troublesome, I tells ya - I never had any issues with such things on my Japanese bikes. But the Laverda cap doesn't seal well. Probably took cues from British bikes.

Fault finding for jumpy gauge needles (magnetic gauges) :

** Smith magnetic instruments rely on a smooth, constant‑speed rotation of the inner cable. Any of these will cause jerky needle movement:

• Dry cable (no lubrication)
• Frayed strands inside
• Sharp bends in routing
• Inner cable too long and bottoming out in the instrument
• Cable outer sheath worn oval

Why it matters: The eddy‑current system inside the gauge is extremely sensitive to rotational irregularities. Even tiny stick‑slip motion in the cable shows up as needle flutter.

Fix:

• Remove inner cable, clean, and lubricate with light graphite grease (never heavy grease).
• Check length: the inner should protrude ~2–3 mm beyond the outer when fully seated.
• Ensure routing has no tight bends—especially near the headstock.
• If the cable is even slightly suspect, replace it. New cables can go bad quickly if kinked during installation.

** If your bike uses a right‑angle drive or a timing‑cover tach drive, wear in the bronze bush or gear lash can cause:

• Pulsing rotation
• Intermittent binding
• Excessive backlash

This produces a rhythmic jump, often proportional to engine speed.


** Rear wheel gearbox (speedo drive) drag or lash: Even NOS drives can have:

• Bronze bush wear
• Gear lash
• Intermittent binding

Fix:

• Spin the drive by hand with the cable removed. It should feel silky, not notchy.
• Check for side play in the worm gear.
• Ensure the wheel spindle isn’t overtightened and distorting the drive housing.

(Suggestions by ChatGPT)

- Knut

Fault finding for a leaky bayonet style gas cap:

* Inspect the rubber seal

Most leaks come from a hardened, shrunken, or uneven gasket.

• Remove the cap and examine the rubber or cork gasket
• Look for flat spots, cracks, or hardening
• Press the gasket with a fingernail: if it doesn’t deform, it’s too hard
• Replace with a new ethanol‑resistant gasket if worn
• Many original bayonet caps used cork gaskets, which shrink with modern fuels. A Viton or nitrile replacement is far more reliable.

* Check cap engagement depth

If the bayonet lugs don’t rotate far enough, the cap won’t compress the seal.

• Insert the cap and rotate it fully into the locked position
• Note whether it feels loose or stops too early
• Compare lug wear: rounded lugs reduce clamping force
• If the filler neck is worn, add a slightly thicker gasket to restore compression
• If the cap only leaks when the tank is full, the engagement depth is usually the culprit.

* Verify filler neck surface

• Run a fingertip around the filler neck lip
• Feel for nicks, dents, or high spots
• Lightly dress imperfections with 600–800 grit paper on a flat block
• Clean thoroughly to remove abrasive dust

* Check venting function

• Blow gently through the vent hole or vent channel
• If blocked, clean with compressed air or a fine wire
• Ensure any one‑way valve (if fitted) moves freely
• If it leaks after riding, especially on warm days, suspect vent blockage.

* Test with a small fuel load

Confirm the repair before filling the tank fully.

• Add a small amount of fuel
• Tilt the bike slightly side to side
• Check for seepage around the cap
• If still leaking, increase gasket thickness by 0.5–1.0 mm

(Suggestions by ChatGPT)

- Knut

Yes, Knut, all this is valid, and good info. And most, if not all, have been checked. Umm, maybe the gears in the right-angle gearbox, assembled from different units, don't play well together. Or something. Maybe the extension of the speedo cable into the housing is 1mm off, not due to me, but I've seen it before, where that is critical. . So now it's fine-toothed comb time, improve my skills and technique. None of this happens on any other bike I've owned (many dozens, over 40 years). They just seem to work, through some miracle. My cable has no kinks, there is no over-tightening at the hub, no lack of lube. But yeah, probably something I've gotta do, may have overlooked. The pleasure of owning a British bike! I wonder why the number of interested parties is diminishing from such joy! Just being snarky, I like the bike and the challenges. But sometimes it gets on my nerves.