We have two 84/85 K100RS - mine, which is daily usage, and my partner's. The second one has been used intermittently, and has now been replaced by an F650GS which is enormous amounts of fun and a better weight for her until F800s drop in price. The second K will be on the market soon - we hope.

The second K had a completely dead battery last time we went to use it. It hadn't spent too long sitting, so we were a bit annoyed about that. There seemed to be a short within the battery, so there was no hope of recovery.

Today, I put my good battery in bike 2 to get it running and check everything.

When I put the second terminal back on, there was a crackle, spark, and the starter motor started up. Apparently, my reactions are quite good.

I triple checked the ignition was off.

Clearly, I'm going to have to do a lot more trouble-shooting than I was hoping for.

Whiel I take a deep breath before pulling things apart, anyone have an idea what the most likely place for a short would be?

Starter relay, my guess. Fusing via a weak charge. 

Battery new or old one recharged?

The best place and most likely place is the starter relay in the relay box under the rear of the tank.
The contacts have a bad habit of welding themselves together when in a dead or almost dead battery situation.
The relay is in the back right of the relay box and can be easily removed and opened with a small screwdriver to pries the plastic case off, you may be able to pries the contacts apart and clean them with a bit of 240 wet/dry sandpaper which will keep it working. There are 2 contacts and both need to be in good condition for it to work correctly.

Ha ha, just found this: https://www.k100-forum.com/t724-starter-motor-fault

Am sallying forth to have a look at the relay right now.

Its an easy fix and if its done in a used one from Motorworks or Motobins is cheap enough. Is easy to fit.

Oh, I love it when a problem is simple. The difficult part was pulling off the fuel lines to remove the tank.

The starter relay contacts were separated when I pulled it apart, but that must have been from my handling it. The contacts were corroded. One was mostly gone, so there's only a small, angled contact. I had to put it back in to check it. I found that an emery board cleaned them admirably.

The bike works so beautifully that with the tank off, a tap on the starter button was enough to make it run and idle for a second just on vapours.

It's possible that wasn't the smartest move, but it was satisfying as hell.

Thanks for the quick response, Rick and Kyle.

I'll be replacing that relay before we sell the bike.

BTW: The battery I put in was out of my bike, so it at least was the known good quantity.
 
[Edited for spelling]

Sweet! Doncha just love it when it's an easy fix? 

Btw, if you replace the relay use a genuine OEM one. I went thru two Brazilian ones before buying a BMW relay and have had no prob's since.