Well nuts. I had my old 1985 K100RS running like a champ. And then I got busy with work, and I let it sit for a week.

Put the key in, ignition on, headlight bright, hit the start button:

CLUNK. Didn't even try to turn over.

I connected my battery charger to the battery, and let it cook for like, 3 hours. Yeah! Headlight super-bright, all lights perfect, hit the starter and CLUNK. Just the sound of the starter hitting the flywheel. Not even trying to crank the engine.

NUTS.

Okay, this is a high-mile bike: at least 90K miles. I know nothing of its past. When I got the bike, I installed a new battery and it fired right up!. Now this!

My biggest concern is that the bike started and ran perfect one week ago. Never a hesitation to start. Maybe the battery is not charged enough? Maybe there was a slow leak in the electrical, that killed the battery?

Starter? Solenoid? Starter brushes? Where do I start?

Anyone with similar experience... HELP!

Yep,
It is your sprag starter clutch, probably sticky with gunge from old oil.
You can bump start it without any bad effects. Get it running, go for a hot fast ride and it may clear it.
If not get some good flushing oil, run with a bump, get it well round it and refill with a semi synthetic oil with a new filter..

Failing this it is a major strip to replace the sprag clutch.
Best of luck......but it worked with mine....87,000 miles.

NEVER attempt to start your bike with a low battery or you will weld the rely points together.
Ajays

Look at your starter motor brushes first and clean out the dust. If brushes are low replace with a kit from www.motobins.co.uk

BadjerJim wrote:
CLUNK. Didn't even try to turn over.

If the starter doesn't turn, I would check the brushes and relay. If the starter spins freely without turning the engine, this is a case of bad sprag clutch.

I concur with Bert, If the sprag clutch is stuck I would expect the starter to whir but not spin the motor over. I would do two things first is check the battery voltage with a meter on the battery whilst the starter is engaged but it is not spinning over and if that is OK I would do the same check on the terminal of the starter motor. If the battery voltage is low then it looks like a weak battery.

If the voltage is low at the starter terminal but OK at the battery it is a high resistance between the battery and the starter or if the voltage at the starter is OK it still could be a poor earth from the battery to the engine i.e. between the engine earth and the battery. Then clean up all the connections from the battery to the relay and then from there to the starter and the engine earth as well plus take a good look at the starter relay contacts. Do your checks with the meter earth grounded to the battery to get the best possible earth ground you can. If there is no voltage problem or earth problem then it is the starter itself and probably the brushes, a good service should set it to rights again.
Regards,
K-BIKE

Sorry for the delay in the reply, and thanks to all who answered!

I replaced the battery ground cable with a nice new fat one. Problem solved. Sometimes you look at the hard stuff before the obvious and easy stuff.