'69 Fastback Ignition problem

Hi everyone, on Sunday I went for a ride on my '69 Fastback and all was ok though I did have a possible fuel problem about half way through felt like a slight blockage then cleared, yesterday I did some maintenance on it drained the carbs etc, I then started it and then turned the lights on at which point it started missing and almost died turned the lights off ran fine, checked the battery 12.9-13.1 volts [not running] I checked the headlight wiring and then removed the bulb and it ran fine, later I went for a ride and it died completely [trailered home] checked the battery again still ok, checked all ignition connections which seem ok, this is a Feb '69 fastback with the points housing behind the motor in the magneto position and fitted with a Boyer Mk3. My suspicion is either a coil or the Mk3 has packed up, I did check for spark and it would spark once but not continue. Today I will try another set of coils then look at the EI, I have another Boyer out of an 850 I believe the EI on the Fastback turns in the opposite direction can I sill substitute the 850 black box?

possm said:

I have another Boyer out of an 850 I believe the EI on the Fastback turns in the opposite direction can I sill substitute the 850 black box?

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The box is the same. The Boyer rotor is set to a different static timing position (Atlas timing hole) for clockwise rotation, that's all.

Pull each connector involved with the issue. Clean them , refit with oxy- guard paste or a tiny dab of Dielectric grease. Make sure the connectors fit tightly when reconnecting. Or buy a new wiring harness with the newer modern material connectors .

possm said:

I then started it and then turned the lights on at which point it started missing and almost died turned the lights off ran fine,

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there's your clue right there...

o0norton0o said:

there's your clue right there...

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Clean up all your lighting circuits and switches and contacts. Look for possible shorts from frayed wires etc.

Could just be a dead battery that cannot feed lights and ign. Or faulty charging circuit.

Don't think it's the battery after the Sunday ride it was still holding 12.9-13.1 volts, tested the coils today which came up ok, but it will still spark once and then cease to spark, tomorrow I guess will be to focus on the EI.

Dead lighting coil on alternator?
It might not be able to produce enough current for both the ingition and the lights at the same time, if you hold a muti meter on battery terminals when engine is running, then switch the lights on, it should still charge at 13.7 volts.

Bernhard said:

if you hold a muti meter on battery terminals when engine is running, then switch the lights on, it should still charge at 13.7 volts.

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Yes, do not neglect this sort of test on the running charging system.

I’d also try connecting the ignition system + and - directly to a known good battery. Will your battery keep the headlight bright for a minute, with the engine stopped?

A battery can test good on voltage alone but not provide the amps to run the ignition, hence you need to do TT's load test.

Bernhard said:

Dead lighting coil on alternator?

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There's no "lighting coil" on the alternator.

One thing to watch out for.

Your 69 would have originally had 12 volt coils.

Using these wired in series with an electronic ignition will massively reduce the quality of your spark as well as prematurely burn out your ignition’s black box.

Most manufacturers specify a total coil resistance of between 3 and 4.5 ohms.

The 12 volt Lucas 17M12 (LU47276) coils that your bike would have had originally are rated at 3.3 to 3.8 ohms.
So stacking two coils in series, that‘s 6.6 to 7.6 ohms in total - which far exceeds the recommendation.

The bike will still work fine for years, but it will burn out your black box eventually.

You may well have addressed this and already fitted the later 6 volt Lucas 17M6 (LU47275) coils, but I wanted you to be aware of this issue, as I’ve come across it a few times.



Check your charging system and battery - I don’t feel you’ve dug deep enough on this.

Looking for three charge states (engine off, idling and at high revs) will help us determine that your charging system is ok:

-1-

-2-

-3-

With a digital multimeter, you’ll probably see quite an erratic meter reading at higher revs.
This is because of interference picked up by the test leads (think of them as aerials attracting interference).
Keeping your leads short and away from everything else on the bike (especially near the coils and spark plug leads) will help stabilise your reading.



The next thing on my list of quick checks would be the ammeter.
With the exception of the horn, the full current draw of tge bike is pulled through this.
The coil in the meter will only last forty years or so before it starts burning out (bloody Lucas and their terrible unreliable electronics again)
You can easily bypass the ammeter (either join the wires together, or if it’s easier jumper the terminals using a wire with crocodile clips on each end)
This will easily rule in or out another questionable area.



The final quick check is around the bullet connector that feeds the battery positive to your headlamp bulb.
These, over years of the headlamp going in and out of the headlight bucket wear over the years, as there is no strain relief on the bullet.
If this connection is bad, it will give you problems, so it’s well worth checking the quality of the bulked and the wires.

The rest of the wiring on the early bikes is pretty rock solid and reliable:

Having good voltage at the source (battery) does not mean you have good voltage at the load (ignition). You need to test both.

Thanks for the replies your help is much appreciated, the bike is fitted with 2x 6 volt coils which test a 2 ohms each, I am no longer able to start the bike so am unable to preform the tests that have been recommended.

If your battery is still showing a decent charge, take a wire from the battery negative directly to the white wire on the Boyer.

Take your fuse out, which will disconnect the NU (brown/blue) that feeds everything else on the bike.

That will at least confirm if your electronic ignition and coils are good or kaput.

L.A.B. said:

There's no "lighting coil" on the alternator.

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I stand corrected, however, the alternator will not switch the light circuit on until the rider/owner switchs on the lights, until then that circuit remains dead.

Bernhard said:

however, the alternator will not switch the light circuit on until the rider/owner switchs on the lights, until then that circuit remains dead.

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All output from the alternator goes to the rectifier (as it's a two wire stator). There's no separate lighting circuit powered from the alternator.

Thanks all, I followed Grants advice today and run a direct line, but no spark. I think the battery was still showing 12.7 volts, after more mucking around I took the battery out of my Atlas which was showing about 12.2 [ hadn't been using the Atlas for 2-3 weeks], checked and found spark and was able to start it though it wasn't happy with lights on, did notice after a bit of further running the zenner heat sink was quite warm? looks like tomorrow I'm off to buy a new battery then perform further charging tests, again thanks for everyone's help.

Up date, purchased, charged and fitted a new battery [YTX12BS] it started first kick but as soon as I turned the lights on it died and again now has no spark, its as though it's tripping a circuit breaker but as far as I'm aware it doesn't have any, I have tried a second black box which came with the Mk3 that I purchased in pieces [which was disassembled due to engine problems] though I'm not 100% sure it does go. where do i go from here? new ignition?

Bad earth?

What voltage do you get between the positive terminal of the battery and the engine case or frame? (Or negative if positive earth) and does this change when you turn the lights on?