Now then I wonder if any one can help?

I Have a 1985 K100, just serviced and it runs very well whilst cool however struggles when I get past 6000rpm and seems to stutter. This I can live with because I have not got it to thrash! However the engine pulls strongly from 1500 rpm untill it is hot, it then takes untill 2500 rpm to start to pull. Also if I close the throttle and come to a standstill, it dies and won't start again untill it has cooled off a bit. The fans are working well and the temp light never comes on. My oil pressure light comes on at tick over at 1000 rpm as well as the battery light however a bit of throttle and they both go out. When choke is out and cold the engine runs at about 1500 rpm. If I put the choke on after riding for a while it increases to about 2500 rpm? Im stumped, any ideas?

Macca

First, your throttle is way too high.
When cold and without the choke, mine is idling at 500 rpm. When hot, it's idling at 1000rpm.
When is the lats time that you checked your gas and air filter?
Have you checked if you have any air leaks (the breather hose is known to crack)

Fuel and air filter changed with service, Breather tube between airbox and Plenham chamber was cracked but put new tube on. Ill take the fairing off and tank to check the other tubes but when my mate did the service he didn't notice anything other than the breather tube mentioned and cracked vacuum caps which have been replaced. The throttle idols at 1000rpm when warmed up after about 5mins but it dies when cold. I have adjusted the choke/fast idle so it ticks over at 1500 rpm when cold on the first turn so I dont have to hold it. This stops the bike from stalling when cold. Im guessing a breach in the airflow somewhere is causeing it to struggle when the revs get high which is likely disintegrated rubber somewhere, however the way it behaves when Hot really is confusing me. What are the symptoms of a Hall sensor failure?

It's easy!

The symptoms of a failing Hall sensor would be the lost of ignition on 2 cylinders when the engine is hot.
The Hall sensors work or fail. They don't give a weak ignition.

mine was ticking over too fast also (see Hot Knees LT !) it took a mechanic to adjust and synchronise the air intake valves for me and now tickover is 950 when warm. only a thought .