I've had two worrying experiences so far. 
      The first was after about an hour ride during a hot afternoon in Florida, kept a more or less constant highway speed for 40+ minutes then some downtown stop and go. During the low speed the bike started making a whining noise and the temp warning light came on. I didn't notice any kind of power loss from the bike but after shutting the bike off it wouldn't start right away, after about 5 minutes it started no sound and no temperature light.  
    After that I did the simple dummy checks, it has fuel, coolant and oil were within levels. 
    I've had issues before with starting the bike when it's hot or right after topping off the gas, Ive sort of decided it's a old bike and I was "starting it wrong" because it's a intermittent problem and it's started again after less than 5 minutes when its given me problems.
    Second time was similar with higher speeds leading up to a longish light. No temperature light just a whining noise from the gas tank? And a repeat no loss in performance or other issues but it needed a short rest and started again with no noise. 

I've tried searching for the problem but might not know the right lingo to find what I'm looking for.  Could the sound I'm hearing be the fuel return line?  


1985 k100 i believe it has the fairing from a k75 

The radiator fan is supposed to start before the temperature bulb lights. No fan?

An elderly K shouldn't need a rest after running—even in Florida —if its coolant is relatively fresh, its radiator fins aren't clogged with debris and its fan works.

Check the fan as I'm betting the wee bugger is probably seized. You don't say which model you have, so I'll skip the strip bit. When you can see the motor on the back of the fan, a good hard wriggle should release the motor cover and give you access to the motor. Check the brushes (or what's left of them! ).

Is anyone here familiar with flat racer? I see a few original parts on eBay but like the idea of a upgrade from stock. https://www.flatracer.com/radiator-fans/flatracer-bmw-k-series-radiator-fan-kit

That price is a rip-off. Search 'spal' on the forum to find out how to do it yourself for way less than half of that cost.

Get the Spal fan and install it in the old frame the original fan was in.  All it takes is cutting the ears off the Spal and And making a single cut in the old frame to expand it to go around the the Spal fan with 2 or 3 pop rivets to hold it in place.  Easy peasy.