1972 R75/5 - Fuse Shorting at Battery

[ryder]

Hello!

A few years ago I purchased a great '72 R75/5 that somebody had tuned up with a digital ignition system (from EnDuraLast & Sachse).

In doing so, they also added a 25amp fuse right at the battery to the main power line which is harnessed under the tank, which I believe goes to the headlamp bucket. (see pic)

Bike has been running GREAT for 2 years. But the other day she started up and rode for about 10 minutes, then suddenly lost all power. The 25A fuse had popped. Went to replace it, the bike started up again for a few minutes and this time fuse popped and died again in idle.

I checked the headlamp bucket and didn't see anything that looked obviously shorted or loose.
It doesn't seem to short when the engine is off but the lights are on, (tested high/low beams, brake lights, signal, horn). It seems to only short once the engine is running.
As far as I could tell the short doesn't happen after a particular action (i.e., it seems intermittent).

Any thoughts for tracking this down?

Cheers

[schrader7032]

Welcome to the forum! The only think I can come up with is to consider putting some kind of light bulb in place of the fuse. I'm guessing it will glow a certain level. Then start taking things off line until you see when one thing cause the bulb to glow very brightly. That would be the offender.

https://w6rec.com/diagnose-blowing-fuses/

[cwf]

After 50 years, it's likely that a soldered connection has failed or the rubber/plastic insulation has worn through somewhere. Start with places that get wet or where the loom passes through a hole into the headlamp. Be careful about moving wires too much as the insulation hardens with age, so, if you've worked on something recently, check there first or if something has been added to the loom it probably hasn't been done to the original standard.

[drpetemurray]

First picture, the diode board harness not connected?

[srankin]

I can't see it very well, in your picture of the front of the alternator is there a wire touching the Philips screw on the added gizmo?

To be honest, I don't like the looks of that whole unit, then again I am NOT familiar with the installation of that ignition system. LOL, you didn't install the system and unfortunately may now have a case of previous owner's disease.

Given the fuse blows when the bike is running, and that this replacement ignition system requires according to my knowledge rewiring of the stock harnesses, I would go over all connections related to it. Who knows what kind of wiring job the previous owner did?

For the picture, did you pull it out from behind or remove it partly? I do see as Pete mentioned the stator plug is off and I see another plug off? Good luck, St.

[richrd]

My first thought under these circumstances is a loose or bare wire that is shorting due to vibration or engine load. Seen this many times with rubber mounted engines that would shift or move when the engine was under load so that letting the clutch out would blow a fuse. or when the engine hit a certain rpm and the vibration frequency would due it.

Oh by the way, this is my first post, I'm going to pick up my first beemr next week and I will be needing your guidance.