Is there a post that gives step by step instructions for cleaning/flushing the cooling system on K-bikes? I would like to know if commercially available products are superior to cleaning vinegar; if the thermostat should be removed for this procedure, etc.  What is the best practice? TIA

I have used Prestone Radiator flush with good results.  I've also used white vinegar.  Drain the coolant, fill with vinegar and run the engine until the fan runs a couple times.  For the Prestone, follow the directions.  After the flush, refill with distilled water and run the engine again.  Drain the distilled water and install your 60/40 coolant mix.  I put a box fan blowing on the headers while I'm running the engine

Have you pulled the radiator to flush the dead bugs from the fins?  I had to soak the radiator from my first 16v K100 to cure a bad overheating problem.  I just pulled the radiator off the bike and put it in a plastic pan with enough white vinegar to cover it.  After an overnight soak I rinsed the fins.  The crap that came out was incredible.

Point-Seven-five wrote:....
Have you pulled the radiator to flush the dead bugs from the fins?  I had to soak the radiator from my first 16v K100 to cure a bad overheating problem.  I just pulled the radiator off the bike and put it in a plastic pan with enough white vinegar to cover it.  After an overnight soak I rinsed the fins.  The crap that came out was incredible.

I think I need to do that to mine. It seems to overheat too quickly when I slow down.

brickrider2 wrote: if the thermostat should be removed for this procedure.

I leave it in place; it will be cleaned during the procedure.

Laitch wrote:

brickrider2 wrote: if the thermostat should be removed for this procedure.

I leave it in place; it will be cleaned during the procedure.

I wouldn't mess with the thermostat. The rubber ring that seals it is very delicate and will tear if you look at it crosswize.

Okay, based on the comments here, I've made some decisions.  I'll buy a couple of gallons of cleaning vinegar and use that rather than a cleaner such as Prestone.  I've de-scaled tea kettles with vinegar and know it's effective on mineral build-up, maybe it will dissolve what I expect is some jelly-like stuff in the radiator that now inhibits cooling.  Given the very low miles on this bike, it obviously sat unused for a long time.  I suspect the coolant was never changed.
I'll flushed the radiator fins with a high speed stream of water, both from the front and rear of the unit. I also brushed off the remains of a surprising amount of insects from my recent tour.  I say surprising because there were very few collected on the windscreen, but the radiator was a whole different situation.  
I'll keep the thermostat in place.  I'd hate to ruin a good-working thermostat for want of a special gasket.

brickrider2 wrote:I'll flushed the radiator fins with a high speed stream of water, both from the front and rear of the unit.

Pointing a high-pressure stream of water at a Brick is not a best practice.

Not a good idea. The one time I did that a stream of flies, crap and radiator fins flew out the back.

I'd suggest an alkaline radiator cleaner rather than an acid like vinegar. I'd say there'd have to be something similar in the US.
https://www.tectaloy.com.au/radiator-additives

Why is an alkaline-based cleaner thought to be superior to vinegar?

brickrider2 wrote:Why is an alkaline-based cleaner thought to be superior to vinegar?

Alkali flush and acid flush have different cleaning properties. Which is used depends upon what is drained from the system. Sometimes both might be needed. Here's the Cummins explanation. The instructions are for heavy-duty vehicle engine maintenance.

The bugs came in from the front, so you want to flush from the rear. 

You want to do it with a minimal pressure, as the fins are not difficult to distort, blocking air flow.  The vinegar soak in the pan not only gets rid of the minerals inside the radiator, but it marinates and tenderizes the bugs in the fins, allowing them to be flushed by just pouring water through the fins.  A bit of gentle tapping on a flat surface helps get the reluctant bugs out.

I realize it's a pain in the azz, but it' the best way I know to do the job.