rotor gone completely dead?

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Sep 26, 2007
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I have been having this strange problem with my Mark III.

After 10 to 15 minutes of riding, it starts to spit and miss - getting progressively worse, especially at low revs.

On short rides (less than 5 miles or this 10 to 15 min threshhold - it runs perfectly.

At first, I thought the issue was related to heat, but that theory got me nowhere.

I then thought the alternator wasn't getting charge to the battery, so that after a few minutes of running, the battery was too weak to run a strong spark. So i put my voltmeter across the alternator leads and there was pretty close to the recommended 9 volts.

A couple of weeks later, and more of the same behavior, I decided to test the alternator output again. Absolutely zero.

This is supposed to mean I should replace the rotor, right?

Is it really as simple as that? I checked the wires from the alternator - no cuts, nicks or anything - nice and clean. (Note - battery is showing 13.5 volts on my meter).

But one strange thing - both leads from my alternator are green & yellow. one of them is supposed to be green & white, no?

Thoughts --- just go ahead and buy a new rotor, or check something else first?

Keith in Encinitas
White will turn yellow over time. However since they both provide power to the rectifier and it matters not which way around ? I think a light bulb for a 100 watt resistor is needed for the test you did. The rotor is the permanent magnet that spins the field coil output is what your testing they are related but get us on track with your thinking.
pkeithkelly said:
This is supposed to mean I should replace the rotor, right?

Thoughts --- just go ahead and buy a new rotor, or check something else first?

Do you mean 'stator'?
I think you mean the stator not the rotor. The rotor has a number of permanent magnets and I seem to recollect that the 'easy' test for the rotor is that it should support its own weight if lifted with a steel screwdriver or similar (if not then the magnets are weak). It could be that one or more of the windings in your stator are breaking-up or shorting - this could be intermittent thus causing your problems.
I assume that the tests you're performing are carried out with the stator wires disconnected from the rest of the bike wiring - if the wires are still connected then the fault could be elsewhere e.g. a short circuit further into the electrical system.
I'd suggest that if your battery still reads 13.5 volts then its not your charging circuit. Do you have a Boyer fitted ? if so it'll work OK down to about 10 volts but the symptoms you describe (if a Boyer is fitted) is the Ignition stator wire broken or arcing. I've also seen this problem with the points lead (from the timing cover to the black box) becoming old and chafing on the crankcases.
If you are reading 13.5 volts at the battery while running, then it is not the charging system. As Gino recommends, check out the Boyer starting with the trigger leads.
Greg (Norbsa48503) has worked out a fix using brass bolts and coax braid that elimimates broken wires due to vibration at the trigger.
testing the alternator - problem with rotor vs stator

thanks for the advice.

I tested the alternator output per the manual, with one little exception - i did not attach a 1 ohmm resistor in parallel across my multimeter (thought it was built in to the multimeter itself, but on second thought... duh!). I will run off to Radio Shack and splurge a couple of bucks to buy a resistor, and retest.

Yes, i was testing with the alternator leads disconnected.
And yes, both the NVT Workshop manual, and the Clymer manual say if you have a zero reading, to replace the rotor, not the stator.

I will also do the other version of the charging test, which is to put my meter on the battery when the bike is off, then when it is on to see if it gets more juice as the revs increase.

Shit - I checked the price of a new Lucas rotor -- $191 at Old Britts (#55202298), and even more at Fair Spares America ($269 for 06-8100). Ouch! I can get a Sparx alternator kit (stator, rotor, rectifier) for $250. If i am going to pay almost that much for just the replacement Lucas rotor, I might as well pay a few extra bucks for the Sparx model, n'est pas?

PS - I am not colorblind. Both my leads from my alternator are green/yellow. I didn't believe it myself at first, so i sliced back the outer rubber covering a couple of inches. They are both the same color - but if it doesn't matter, no big deal (?)

Anybody been in this bind ? replace rotor only vs the whole alternator with Sparx for a few bucks extra (the extra bucks doesn't bother me) - just whether the Sparx really is any better than stock for a Mark III

If the battery is showing 13.5 volts then the charging system is not at fault.
The misfire must be coming from another source, ie dying sparkplug etc.
Re: testing the alternator - problem with rotor vs stator

pkeithkelly said:
And yes, both the NVT Workshop manual, and the Clymer manual say if you have a zero reading, to replace the rotor, not the stator.

The Norton factory manuals don't actually say that.

But I do agree that the information given is misleading, as it does say:


"If the reading is low, check the rotor by substitution.
Zero reading indicates open-circuited coil(s)."


If a "low" reading is taken? = Replace the rotor.

As there are no "coils" in the rotor, then the second sentence is actually referring to the stator being at fault (if a Zero reading is indicated?) not the rotor.

I notice that the Clymer author appears to have borrowed the factory manual information, and copied it in his own words, but he has not written it up correctly, probably because he also misunderstood it?
To test the battery do this: Motors not running turn on the head light for three minutes leave it on check DC volts across the battery leads. This removes the surface charge and tells the real tale. Report back. No other data is any good.
Since many of us have the old stock stuff, Rotor and Stator on board and they work fine stop, take some time ,save the money gun for later.
Get yourself a hand held or a panel AMP meter they are very cheap at electric supply house's around town. Mine looked 8-10 years old cost five bucks in the bargain bin. + to - 10 AMP's is plenty. Now hook it up as if it were an inline fuse for the battery. Now you can get a reading that means something. Start the bike nothing on but the engine at idle you might get a middling reading but as you rev the bike you should see some positive AMP's being put out at 3500 RPM you should have all you need 3-4. with the lights on running at 3500 you still need at least one Amp to charge the battery. If it reads backwards flip the wires around on the meter. AMP's charge a battery not volts. A volt meter used the same way can tell you if the Zenor is shutting down the charging system at or near 14 volts to protect the battery .
got a good test - alternator is putting out

Got the test done on the alternator leads. It is putting out exactly 9 volts at 3000 RPM, per the spec.

So unless it is acting up intermittently, maybe the problem is elsewhere. I will do the additional tests, and do the ultimate - which is to ride it for 15 minutes, when the crappy behavior kicks in, and check the state of the battery charge right away.

That will tell me if my theory is valid: that the battery is discharging after a few minutes of riding, and there's not enough juice left in it to provide a strong spark (yes i have a Boyer).

Thanks for all the suggestions - just wish i wasn't such an idiot about electrical stuff.

Take a hint, take off the points cover, start the bike, shake the wires coming from the boyer pickups. Its obviously not a charging problem if you are constantly reading 13.5 volts.
the mystery continues....

took the bike to a shop nearby 2 guys who know their way around Brit bikes, including 1 who has worked on Nortons for 30 plus years (and rides one every day).

They did all the electrical tests, and checked the carb. Could find nothing wrong. My problem (missing, spitting, and lunging at low revs (between 2000 and 3000 RPM)), normally doesn't start until after i've been riding for 10 or 15 miles.

the first sigh of the problem is a brief spit back on deceleration, just a bit, then it quickly gets worse over the next 5 min of riding, until it is chugging and rough (until i can get the revs over 4000 RPM. It's even tough starting off from a red light, with all the missing and spitting going on. Have to rev it way up just to get moving.

But for the first 10 or 15 miles of riding, it's perfect.

I have: (for 1975 Mark 3)
- single Mikuni 34 mn (just 6 months old)
- Boyer (analog, Mark 3 model - installed by prev owner)
- plenty of compression (600 miles into a valve job)

What i've done:
- checked charging rate several times (voltage and amps), and all other electrical tests (rectifier, coils, etc.) - all in spec
- swapped batteries a couple of times
- swapped with a fresh set of spark plugs

When this happened a couple of days ago, after the shop couldn't find anything wrong earlier in the day, I stopped the bike. Neither coil was hot, and i couldn't see anything going on. (It idles at 1,000 RPM just fine - but trying to get the revs up is what gets it spitting and missing).

So it's not an intermittent problem. It is consistent - starts after 10 or 15 miles of riding.

Am i doomed to spending the rest of my life on short little rides?


Thanks. Keith in Encinitas
Why not borrow a fully charged battery and take it in a backpack on your next ride. When the symptoms occur change batteries and see if the bike runs OK. This will at least narrow the options down to charging, which you originally suspected, or something ignition related. I'm surprised the chaps who 'know their way around British bikes' didn't take the bike (and an electrical meter) for a spin to replicate the problem. Electrical connections and joins can alter their resistance when hot. I ended up soldering some key wires together rather than relying on male-female snap connector,s for similar reasons that you describe.
Perseverance will solve your problem
You need to determine if it is electrical or fuel or air. Isn't that something it can only be one of the three. Now when your paying professionals to do the work and you are trying to do some testing yourself it can get frustrating.
But now you are soliciting the opinions of many about what it could be. Everyone of us is had some strange problems like this you are not alone. You seem to have some issues about reveling the raw data of the tests you have done that is not going to simplify your solving this problem. Even though you will get answers like the only plug I will run in my bike are Champion and that has nothing to do with anything you have to post the data if you want to get led to the answers.
Lets start by just running around some found answers that come up before when things like this come up. Now the purpose of this is to illustrate how very silly these problems can be and how easy it can be to over look them even using professionals.
Dead cell in the battery. (previously not found because no one had the proper load tester to find it.)
Broken Boyer wires in the point cavity (not found previously because no one believed it.)
The wires from the the old points location have crossed the AC wires from the alternator on their way back to the Boyer box. (No way that could be it.)
Many of the negative wires in the harness are found to be black as night way up inside the covering.( Well at least you can't see this one easy. )
There is no good ground wire from the motor to the common ground. But the factory never had one. (That don't mean you don't need one.)
The coils are hit. (But it runs great for ten minutes and there dead cold.)
The key switch is found to be the problem.(well it runs great for ten minutes)
The kill switch is found to be the problem.( It runs great for ten minutes)
The ground wire from the black Boyer box it not hooked to the common ground. You starting to get it.
The positive terminal of the battery is not hooked to the common ground.
The ground wires in the harness have broken were they hooked into the frame and were never tied back to the common ground.
The old rectifier has lost it's ground to the frame.( best spot for a common ground.)
The female bullet connectors under the tank or the wonderful Mark three connectors have failed (dang things only last 30 years what junk)
I replaced the old rectifier with a new better one but didn't clean the contact on the Zener Diode (there you went and lost your ground again.)
I bought a brand new box so as to get rid of the rectifier and the zenor ( but I didn't hook the ground wire up to-you guessed it the common ground.
Your favorite former owner did not follow the directions for the coil wiring. Yep he didn't hook the last coil into (the common ground)
The worst thing to find (just ask a Norton owner) two problems at once both causing the same problems with the running of the bike. I.E. the key switch and the kill switch both are starting to take a dump (but not quite at the same time)
Now I am sure that I can think of more solved problems. ( but why should I have all the fun) come on Owners tell us all your (A kill ies heels)
Ludwig, real good ones.
Petcocks test : remove the line leave the spigot, now you ought to undo the spigot than remove it from the line than put it back on. Get a clean coffee can size container. Turn on the petcock. run for one minute, there should be no ripples in the stream just a solid round stream of gas. You might be well into the minute before seeing that it's gone wrong. You could find water and or rust, tank liner, petcock parts.
The 2MC is one that slipped my mind. Were these still on the mark three's? I don't own one can ya tell? If they are bad they cause a constant drain on the battery and if good they operate like a small battery boosting the DC output.
I am thinking that if the bike were mine I would be looking at all the connectors add at the time the Boyer was done many people don't crimp well let alone shrink wrap for strain relief.
And here's another over looked one when was the last time you renewed the rubber line for the balance tube on a stock set of Amals?
Ok here is my hard one to find. My 850 Roadster used to misfire badly but only every now and again. I done all the usual checks and only determined it was electrical. Swapped Boyers with all my mates, still the same. To cut a very long story short, I rode the bike slowly with a mate sitting on the back and bouncing up and down and another mate running beside looking for possible problems, it was an extremely funny sight!
Anyway, turns out the rear brake light switch wires were bared and would earth on the frame rail causing an extreme misfire and sometimes stopping the engine. Moral is, keep looking. Try cleaning the bike, I find potential problems while doing the detail work sometimes.

I finally decided to throw money at the issue. I had traced all the connections and wires all over the place, but to no avail.

I bought a Sparx 3 phase alternator kit, with the regulator. Installed it and all was good.

Maybe i had a bad regulator all along, maybe it was the stator...? Whatever the cause - problem solved.

Appreciate all the help.

Come on Owners tell us all your (A kill ies heels) -Right!

norbsa48503 said:
You need to .......
Now I am sure that I can think of more solved problems. ( but why should I have all the fun) come on Owners tell us all your (A kill ies heels)

A related one. My 97 Golf would drive fine from home to work everyday and anyday (20 minutes). If I drove it 5-10 minutes father to shop on the way home, it would drive exactly as this problem - missing, can't accelerate, driving like crap. It would idle fine though.
If I left it in parked for 5-10 mins. It would run fine but then start to run bad after 5-10mins.
Electrical - no?
Battery was fine, headlights fine, can't hear any relays under the dash cutting in or out, no relays under dash hot, no fuses blown.
I checked sensors under the hood, all seemed to be working fine, nothing came up. Fuel pump was on - got gas.
Checked the plugs. They all looked O.K. nothing way off.
The answer - swap out the only simple thing left as learned by working on motorcycles - swap out questionable with good.
It starts to run bad (on cue at 27 mins), stop fast and swap two coil wires with spares - nothing!
Drive a bit – still running like crap, immediately stop and swap next two coil wires – gotcha!
One coil wire took ~27mins of run time to fail. It would idle fine.
Next would be hooking up to the garage diagnostic sensors which may have found it, but I was having way too much fun. :roll:

I like to swap out known good parts. Also, even if you just disconnect the whole wiring system from the power wire to the ignition, and just run the ignition with a single wire from the battery – as the battery will last a long time if you run just the ignition. No switches required, take the inline battery fuse out to stop, with the fuse in you’re runnin’.
This eliminates with one try the entire electrical system with all its connectors and switches and charging and splices and c**p.
You have isolated simple power from the battery, to ignition and fuel (carbs and plugs) etc.
The reduction of variables breaks a difficult job into a simple one.

I remember someone had a thread – what you would keep in you tool kit?
Simple - a length of 14 or 16 gauge wire – maybe 6 feet. It works better than running your ignition with a Norton choke cable core as a power wire. DAMHIKT, BIKT
Also, if you lay your plugs on the head and get a good spark it does not mean they are any good. I eventually found a bad plug as it slipped from the head and I could see many sparks way up inside the insulator. Many spark paths do not a good plug make.
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