I have been riding my k and enjoying every minute. On my last ride the bike ran well until I was returning home, at 50mph and above 3000 rpm,  the bike started to misfire and stalled, when running at normal traffic speed of 20/30 mph the bike ran fine. I did have my lights on, and when I got back to the garage, I put the battery back on charge.

Is the fuel filter clogged and passing insufficient fuel? 
You seem to have determined the source of the poor running is electrical, but why?  Have you had this problem in the past?  
Which model are you riding?  

What is the condition of the four pin tank connector?  They are known to cause problems when running from the vibrations and dirty/loose pins.

Point-Seven-five wrote:What is the condition of the four pin tank connector?  They are known to cause problems when running from the vibrations and dirty/loose pins.

The connection is good, I keep it clean and tight fitting.

brickrider2 wrote:Is the fuel filter clogged and passing insufficient fuel? 
You seem to have determined the source of the poor running is electrical, but why?  Have you had this problem in the past?  
Which model are you riding?  

The fuel system has been replaced, new mounts and fuel pump and new filters. My bike is a 1988 K100RT. The problem has been there since I got the bike, recently it has got worse.

Starter motor cleaning? Ignition switch cleaning? Temp sensor condition?

Stumpy wrote:The problem has been there since I got the bike, recently it has got worse.

Have you checked the condition and attachment of the z-hose breather that runs from the crankcase to the air box on the left side of the engine. Have you checked the rubber intake manifolds on the throttle bodies for leaks using carb cleaner or propane while the engine is running? An engine with air leaks might perform acceptably at low revs but lean-out and stumble under load.

Fuel filter is the first thing I would check and if you don't know when it was last replaced then replace it.

No one has got close, it is the regulator/ rectifier, the brushes are knackered. 
At low revs the voltage was 13.7 at high revs the voltage was 12.9. I have just put the new one on the bike,I have not yet been out to check it but with everything else that I have replaced and repaired, I am confident that this was the problem.

Twas my next reply. Approx. how many thousand miles on that VR?

volador wrote:Twas my next reply. Approx. how many thousand miles on that VR?

My bike has 50410 miles, they look like the original set. I am going to clean the housing and replace the brushes for future use.

I'm a firm believer that the voltage regulator can be the source of running problems and shouldn't be overlooked when all else fails.  I had a 16 valve K100 that would die when coming down to idle from a high speed run.  After over a year of checking everything in the engine controls the problem was solved when the voltage regulator was replaced.

The engine controls rely on adequate electrical power.  When all else fails to correct a driveability issue, replace the voltage regulator.  Fortunately, they are not terribly expensive or difficult to replace.