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Discussion Starter · #1 ·
I headed across town today to give my mom a Mother's Day card and my 78 T140 just quit as I approached a stop sign. It started right back up but then happened again and got worse and worse after a few miles. It was like the key was suddenly switched off. I eventually realized that it would run fine if I left the headlight off. By the time I got back home the headlight would kill it as if it were a kill switch. Battery is a 3 year old Carquest China special. It shows 12.2 volts and the horn sounds pretty normal. I built my own air cleaners since I live on a dirt road and the originals seemed to seal poorly so I mounted the zener on a frame gusset- was that bad? It is normally cool where I live so I didn't give heat issues much thought. What have I done? Or could a lame battery make it act like that?
 

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12.2 battery Voltage means its discharged or damaged.
With the motor running the alternator should supply adequate power.
I think the battery needs to be charged for ten hours.
A battery check will confirm if the battery is naff:
After charging, engine not running, ignition and headlight on for ten minutes...battery Voltage should not fall below 12V.
 

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Check all the connectors inside the headlamp shell - they are notorious for shorting out.

The same situation happened to me very recently.

Thankfully the fused connector in mine melted before killing the wiring loom.

Also, the battery should be checked as mentioned above.
 

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dead batt - try recharging and see if it holds it.
if not - it's time for a new one.
is the zener in it's new position WELL grounded ?
 

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The exact same thing happened to me this past summer. after that happened twice the bike went dead about 30 miles outside of town. After finding that the battery was fine and that the wires around the coils were fine I went through the wiring and found a short.
 

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it's a common enough event - the only thing that makes it different
is that the original poster indicated 12.2 Volts at his battery. = suspect battery & not an electrical short.
 

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Discussion Starter · #8 ·
I have a friend with a battery shop and dropped the battery by, he checked the voltage which he agreed was slightly low, he trickle charged it for a few hours, tested it again and then load tested it and pronounced it sound. I am thinking the short in the headlight shell idea is what I'll pursue next. I ground back the paint when I mounted the zener so I am confident the ground is good. Thanks fellas, I hope I can pay it forward sometime.
 

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I headed across town today to give my mom a Mother's Day card and my 78 T140 just quit as I approached a stop sign. It started right back up but then happened again and got worse and worse after a few miles. It was like the key was suddenly switched off. I eventually realized that it would run fine if I left the headlight off. By the time I got back home the headlight would kill it as if it were a kill switch. Battery is a 3 year old Carquest China special. It shows 12.2 volts and the horn sounds pretty normal. I built my own air cleaners since I live on a dirt road and the originals seemed to seal poorly so I mounted the zener on a frame gusset- was that bad? It is normally cool where I live so I didn't give heat issues much thought. What have I done? Or could a lame battery make it act like that?
When I had the same symptoms on my T140, it turned out to be a dodgy bullet connector in a wire from the alternator to the rectifier. It was just behind the cylinders above the gearbox. Might be worth a check.
 

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The zener would be well enough grounded by the ground wire attached to its base.
For the zener to survive,it needs to be mounted on a heat sink.The air filter case was also an aluminium heat sink to cool the zener.
A piece of aluminium sheet 1/8" thick and about 5" square would make a good enough heat sink.Good contact between the zener base and the sheet helps.So does a little bit of thermal grease.
 

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I have experienced a similar problem with the bike not running with the headlight switched on. It was a charging problem, eventually traced to a faulty alternator stator; one of the windings was shorted to ground internally. It seemed to be able to generate enough power to run the ignition, but not the lights as well.

Run the engine and measure the voltage on the battery, it should get up to approx 14.5v.

Take a look at the rectifier, is it the original 'finned' device? make sure it hasnt been overtightened and the connections broken, and that it is well grounded on it's mounting.

In order to reduce the number of things that could stop the engine, I have also wired out the 'kill' switch.
 

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I had that happen.
The battery was flat.
Charging system could handle firing the plugs, but the headlight and signal lights killed the deal.
 

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Discussion Starter · #13 ·
I figured it out and fixed several potential things along the way. The fuse holder was failing as vibration apparently caused the metal contact at one end to retract into its surrounding plastic, gradually weakening the connection. It finally failed outright ( in the driveway) and I was able to pinpoint the problem.Pretty weird, huh? I also moved the zener to the aluminum frame of my new air cleaner, found some marginal connections, changed a damaged plug wire etc. Thanks for all the input!
 

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The fuse holder was failing as vibration apparently caused the metal contact at one end to retract into its surrounding plastic, gradually weakening the connection.
Do yourself a real favour and get rid of the spring loaded, tube type fuse holder. Pin type fuse holders will not give you these connection problems.

You've had a taste of intermittent wiring problems here, so while you're at it take some time out and check that rotten old wiring harness for any signs of rubbing and or actual bare wires.

Locktite kid has also reminded you that the countless bullet connections on these old tarts are open circuits just waiting to happen.

Take heed of Mr Pete's advice about some thermal paste on the Zener. RR
 
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