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'68 T120 with 3 phase alternator and Boyer rectifier.

While I was setting up the carbs for approx 40 minutes, lots of idle running, engine dying, kickstart again... I noticed the green ignition light stopped lighting up when the engine died during the carb tuning (when the engine stalled). It looks like the battery was drained flat, should it be? I'd of expected a 3 phase alternator to handle running at idle and charging the battery all day long or am I expecting too much and what I'm experiencing is expected?

Thanks in advance.
 

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Ho oskar866, Next step is charge battery fully. Let set a few hours or overnight. Check voltage key off. It should be 12.6v or higher. What is voltage?

Key on, motor not running, what is voltage at idle lights off, lights on?

Leaving meter hooked up to battery, motor stopped, turn on key & lights. What is voltage? Keeping key & lights on wait 3 min, record voltage, wait 5 min record voltage.

Charge battery again. Wait a hour or so. What is voltage?

Rev to 1500, 3000, 4000, read, record voltage lights off, lights on?

Record all this & report back with findings.

Don't make any repairs before doing the voltage tests. Idea is to get base line test how bike is now. Then you'll be able to see the difference any repairs make. Do not fall into trap of buying & installing parts/battery until you make these simple tests.

If you don't have volt meter, you'll need to buy or borrow one. Note: some/most of the really cheap meters cannot read voltage with motor running. The voltage reading will go crazy motor running, but fine motor stopped. Maybe UK members can advise on meter sold in UK that works good.

The idea is to start with charged battery. Perform shade tree load test of battery. Charge battery again to get fresh start. Start motor & test output of alternator. Point is you must have fully charged good battery to test output of alternator. Low charge or weak/failed battery will skew output voltage test & not give reliable result.
Don
 

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Maybe UK members can advise on meter sold in UK that works good.
It's my understanding that a analog meter will work. That the problem with a modern digital is it tries to keep up with the wild fluctuations of an old Brit bike at idle.
 

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Fluke make very good meters and some will read amperage as well as volts. They are digital and expect to pay £60. Expensive for a hobby but it will last many years.
If you have a 3 phase stator, it might be that your rotor magnetism is weak. I had a rotor that worked well for 40 years before the battery began losing charge every ride. The rotor magnetism was almost all gone.
 

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Hi oldgoldie, Good point!! Analog works perfectly for old Triumphs.

Kind of like a clock with hands.... Can anyone read needle type meter anymore? Just kidding of course. I still have analog, but I'm getting to blind to read it!
Don
 

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Hi,
Note: some/most of the really cheap meters cannot read voltage with motor running. The voltage reading will go crazy motor running, but fine motor stopped.
It's my understanding that a analog meter will work. That the problem with a modern digital is it tries to keep up with the wild fluctuations of an old Brit bike at idle.
Depends what you're measuring. Battery present and measuring DC Volts, there should be minimal "fluctuation", that's one reason for having a battery in the first place ...; "wild fluctuations" is either a problem component or - most often - a meter without emi (electro-magnetic interference) protection, HT then causing interference to the meter.

Afaict, lack of emi protection shouldn't be a problem in GB and probably the wider EU - despite using cheap meters for years, certainly I wasn't even aware there was a problem 'til I read US posters on internet forums.

Hth.

Regards,
 

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Digital meters use sampling of an analog to digital converter. Points opening and closing changes the battery voltage because of the the load change. Plus, there is a back EMF in the coil when the points open putting a spike on the line. Cheap ones likely do not have much filtering so every time the voltage changes the reading changes. Analog meters have physical damping.
And, No, a Lucas three phase generator will not keep up with the power drain at idle. You mentioned a Boyer rectifier, what ignition? Unless it an electronic that times the coil power to the RPM it is drawing a lot of wasted current. Points have a fixed dwell which is in crank degrees. It has to have enough so the coil charges at high RPM which means it is powering the coils far too long at idle. The old Boyer ignition turned the coil back on by a fixed time after firing. Again wasting power at low RPM.
 

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I have a Fluke meter, Simpson 260 analog from the 1950's and a hand held analog volt meter. I was working on a US 4 cylinder with points ignition. It has a ballast resistor on the 12 volts to the coil that drops it to 9 volts when running. The voltage fluctuates between 14.2 and 9 volts as the points open and close.The fluke showed an incorrect steady 12 volts because of averaging. The hand held analog meter gave a correct fluctuating reading.In this case I need to know the actual operating voltage when the points were closed..
 

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Hi All, I'm talking about the cheapo meter from Harbor Freight. The occasionally will give free with any purchase. I'll get a cheap thing so meter costs less than $1.00 with tax. We got them often. Used at work for residual current at rest testing. If you forgot to bypass meter & opened door would smoke the meter. Fluke fuses are costly.

Used to be $4.00 when not on giveaway. I see price increased. I'm sure same low quality.


You start motor & the reading goes wild. My Extech meter it reads perfectly stable with exact same hook up.

I gave my last spare chepo to my son. Then smoked my last one. No promotionals recently so I may have to buy one! I don't want to smoke my good meter.
Don
 
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