I have a problem starting my 1986 K100 RS. I recently replaced the O-rings on the fuel injectors, then started the bike in the workshop (started easily), ran it for a few minutes to check for fuel leaks, and all was OK. When I next tried to start the bike a few weeks later, the starter would crank the engine for one slow turn then disengage on each press of the starter button. If the starter button was held down, it would cycle through one slow turn, disengage, then another slow turn, disengage and so on. On a single occasion, it spun normally for a few turns, almost started, but the reverted to the slow single turns. Plenty of fuel, fuel pump running and the battery fully charged (13.4 volts).
Any ideas from the forum greatly appreciate.

I'd still suspect the battery. Charge it fully overnight, then leave it for the following day to disperse the surface charge. That willl give you a better idea of the retained voltage. If it's above about 12.8 volts, the battery should (in theory) be okay. Try starting the bike and after a few attempts, check the battery again. Contrary to common opinion, they can just die overnight with one or more lead plates collapsing for no apparent reason.

battery can read correct voltage and still be shot , try and jump start off a car etc just to make sure .

It sure sounds like a voltage problem to me.
Check the voltage at the battery terminals when it is cranking, it should be above 9.2 volts. If the cranking voltage is almost the same as standing voltage when it is slow to crank check the earth connections at the gearbox and the terminals on the starter relay and battery. Clean them back to bare metal that is no corrosion.
If the voltage is below 9.2 volts the ignition and engine fuel injection modules will not work. A good battery will read over 11.5 volts when cranking. The Ks are very fussy about battery voltage especially the ABS ones. Low battery voltage can and often does cause the starter relay to weld the contacts causing a bit of panic until the battery can be disconnected.

+1 on cleaning the battery and ground connections.  Do that before any other troubleshooting activities. 

If cleaning the connections doesn't fix the problem, pull the battery and have it load tested.

If the battery checks okay and the connections are clean and tight, the problem may be burned contacts in the start relay.

If the start relay is good, pull the spark plugs and see how the starter spins the engine.

JET wrote:I have a problem starting my 1986 K100 RS. I recently replaced the O-rings on the fuel injectors, then started the bike in the workshop (started easily), ran it for a few minutes to check for fuel leaks, and all was OK. When I next tried to start the bike a few weeks later, the starter would crank the engine for one slow turn then disengage on each press of the starter button. If the starter button was held down, it would cycle through one slow turn, disengage, then another slow turn, disengage and so on. On a single occasion, it spun normally for a few turns, almost started, but the reverted to the slow single turns. Plenty of fuel, fuel pump running and the battery fully charged (13.4 volts).
Any ideas from the forum greatly appreciate.

Thanks for the advice everyone.
I'll work through the suggestions and see how I go.
Much appreciated.

One last thing.  Have you checked the brushes in the starter motor?  They can be worn to where they don't make good, reliable contact anymore.

https://www.kforum-tech.com/Downloads/electrical/diagrams/starter-1.jpg
https://www.k100-forum.com/t2342-starter-motor-cleaning-tutorial
Regards Martin.

Measure the voltage on the battery when you try to start, a bad battery exposes when the voltage drops under 10.
Also clean the earthline on the gearbox (left side).