I got a weird new one for the Lucas tshoot fans!!

So I buttoned my bike up again and went to give my dad a helping with his bonneville. Nice 68' pretty original. We took te head off for some top end work. But that's not my story here...

On the way there I took the crest and all was well. Bike running better than ever now.

On the way back all well as well. Until I got to hidden springs tunnel. I decided to turn my headlight on. Turned it on and everything was ok. I decided to leave it on the rest of the ride. Now a few turns later I get a pop and the ignition missed a few times. Feels like its gonna die.

I fiddle with the ignition switch and get some ignition but its on off. I must look funny on Angeles cret at 65mph one hand fiddling with Tyne key and the other going throttle on and popping off.

I then turn the headlight off and the rest of the ride is good.

almost home I decided to try again on a long straightaway.

I turn on the light and count to 30 at 60mph and then the ignition starts missing and stuff.

I'm thinking there is too much draw with the light on so maybe there's a short or something? I required things a few times so damn if this is an easy find!!!


Any ideas to keep me on the easy route before I end up looking at a thousand pieces of motorcycle on the garage floor?

It sounds like your charging system is only putting out enough current to run the ignition but not the headlight also. Likely problems would be a bad rectifier or a weak rotor magnet. If the rotor has ever rubbed on the stator it will be demagnetized.
A bad zener or stator could be possible but they would be down the list. Jim.

Check bad connection of one of the Alternator charge wires. Even baseline Lucas can keep up with hi beam and brake light on at hwy speed/3000+ rpm. Do check carb sealing after such rough running and expect to loosen some other electrical connection while fiddling around for the current one, ugh.

I"ve played mechanical on the fly plenty of times, getting burnt and shocked but often enough ID's the fault and fixing on the fly or pulling over to do it better.
Always reminds me of early flight days with mechanics out on a wing nursing it along.

If you guys saw my charging system you would laugh.

I think you have in my earlier primary leak troubleshooting.

I'll try and dig up a picture.

I had an ignition switch fail caused a problem like that. It would run fine until I put the key to Ign+lights position, then it would die.

I found this out a few miles from home at night, I knew the road well, and got home by running for a bit without lights, then a quick turn of the key for blast of lights (and no power) for the corners. It was a bit hairy.

Symptoms a bit different from yours though which does sound like insufficient power from the charging circuit, or I guess several other posibilies!

Here's my stator. It's probably the ugliest thing on my bike!!

https://picasaweb.google.com/lh/photo/G ... feat=email

That looks pretty sad, but with stators looks don't mean much.
Check the rotor by seeing if the magnet will stick to a 3/4 inch wrench strong enough to pick it up.

Sounds like ignition switch. The fact that you could fiddle with key and improve it best indication. Take apart and clean or better yet replace.

Anything along current paths can do this but key fobs flapping in wind blast can accelerate fault in key switch. Kill button is another weak link to catch us out. I am a pussy at times not wanting to travel by myself d/t wanting company and emotional support or entertainment stuck away from home. Two rides ago a coil wire fell off and got HT lead shocked feeling around but also pleased at full firing so stopped and slipped terminal back on and so far so good so will tempt fate on a work commute later toady. Every ride is a leap of faith and tempting fates for me.

This is classic.

1. Either weak battery, load test it out of the bike.
(more than likely)

2. Charging is insufficient, check voltages DC at battery at different rpm to start with. If no change is present at rpm changes, check AC output before regulator from stator leads. If AC voltage is present then bad reg/rec. If no AC voltage, then bad stator or broken/bad leads.
(Definitely possible)

3. High load present. DC amp probes are not too common so check for a large drop in voltage when lights are turned on. Troubleshoot and isolate if needed. This scenario would also include a bad switch.
(not likely, but possible)

Check all your connections and remember, the stator feeds the battery and the battery feeds the load (ignition, lighting, switches, etc).

What headlight? 48 watt original candle? 60 watt H4? Could be as simple as a high resistance connection at the batteyr. fuse, switch, etc. supplies adequate voltage to ignition but when lighting current draw is added, it causes the voltage to go very low. As a test, put the the battery on a charger till it's topped up, then immediately go ride/test again. If symptoms persist, it's not the charging system. Can you temp wire a small DMM to read at speed? Or just do it in the shop. Check voltage at the battery while running, then at 3000 rpm, then add light. Then check at ignition unit. I agree, ignition switch is cheesy, high likelyhood. I fiddled mine down off a mountain pass during a group ride in the rain.. loose rivets were the cause

As always lots of great comments.

I had to drop my dads triumph head off at dean collinsons today so I asked him too. He pretty much said a culmination of what you guys said. Fort thought he had was if my battery had 8 hr charge before running (new battery). I said yes, then he said " what kind of was light you have?". I don't recall, then he said that if the headlight draws a lot and he electronic ignition in tandem then it may not have enough charge to keep up.

He said the same things you Gus are saying even down to the charging test in the garage under rpm load.


I'm gonna have to yet this out and report back. If its just the switch faulty I'll look at that too!

We never had this problem with acetylene lighting .

Those infernal Lucas switches can be a bitch .



like computors .



these come in old pommy cars , and are very good.



these ones are absolute garbage .If you have one , Post it to someone you wish to drive insane .

ALSO , check insulation on wires through grommets ( split / ) & around steering head. 7 under tank etc for crushing / chaffeing .

Happened to me twice. Once on my MK111 and once on a 73 Interstate I used to own. In both cases it was a faulty ignition switch.

I read this thread as Im having some confused moments on the switch wiring.
Bike is a 74 looks standard. Ignition switch is stamped 35351a. Four poles. White
plastic base. All poles have two male blades. Headlamp toggle is on\off only.
Has stock lucas alloy handlebar switches ( disassembled and cleaned contracts
they dont seem to be the problem).
Back of ignition switch is numbered 1-4. They are as per book diagram color
position wise. 1 is brown blue (battery) 2 is white 3 is brown green (general lighting)
4 is blue yellow ( supposed to feed headlamp switch for main headlamp)

In the parking position brown blue is hot and 4 blue yellow is hot. So if you
turn on the headlamp toggle you get hi or low as selected by the bar switch.
You get the headlamp parking lamp on with or without the main headlamp.
You get no tail or instrument lamps.
In the off position you have only the brown blue post hot.
In the daytime run position you have 1 brown blue and 2 white hot only. No
lights anywhere.
In the nighttime run position you have 1 brown blue and 2 white and 3 brown green hot.
You have tail and instrument lamps but no main lamp. You can get the main lamp by
hitting the flash button. The flash button feeds off the white wire and the mains
normally feed off the blue yellow from the ignition switch through the headlamp
switch down the blue to the handle bar switch. From there it either goes blue white
or blue red depending on HI-LO selection of handlebar switch.
The problem is that the blue and yellow isnt hot when Im in nighttime mode.
It is only hot when the switch is in park mode. Ive followed all circuits and the
color codes are correctly joined. If I put the blue yellow on the male with the
brown green on post #3 then Im ok EXCEPT then I have no parking lights.

It must be as obvious as cancer but Im hot getting it.
Comments please!

This is probably NOT your problem but...

When I'd completely rewired my bike, replaced all the Lucas bits except the taillight and the handlebar switches, and soldered everything to my satisfaction (with proper insulation and stress-relief), I took a test ride and the bike was running great. Til I hit the high beam switch. Immediate death.

Long story short, a hot wire was grounding inside the headlight shell. Lots of connections in there, and lots of room to move about and possibly contact that nice big, grounded metal shell.

Best of luck.

It is both a delight and a weak crutch to blame DPO but...
Part # on the master (ignition) switch is 35351A marked Lucas England.
However Andover says
1974 191269

but to use the 1976 MkIII>

NORTON 850 MK III - 1975
Item: MASTER SWITCH(NO LOCK) 12.1015
Part Number: 066395

I m wondering if nothing is actually wrong just that the switch doesnt connect
correctly inside, ie it isnt the exact switch as I should have.

Lots of good stuff to check. Till now I have just refrained from blasting the light. I'm gonna go through the options and try to get this thing isolated.

Thanks guys, your a great bunch. I appreciate it.