Hi I had an issue today with my 1990 K100 Lt Just after leaving work and about 5mins after starting it up (so not reached norm temp) I lost power under 2000 rpm. It felt like a bad earth or fuel starvation. When i tried to accelerate it would not have it but did not cut out but was running really rough. I the turned it off and after about 2 mins started it and it was fine i then did 30 miles on it not a problem! Just now i went out and same thing! 
                                                                                                                    Last week i noted it sounded rough once ant so this week end i serviced it and did a few bits to ir as follows:
Oil and oil filter
Fuel filter (arrow facing away from pump)  
Air filter
4 bosch plugs (the same as what was in it before) 
New rear brake disk and final drive spline lube
I have fitted a temporary new key ignition (did that Tonight as i taught the problem today was a faulty ignition) 
New clutch cable  (last month I replaced the rear main seal and fitted a new clutch and did main spline lube) (I have probably done 1000 miles since then)     

I am wondering if it could have lost fuel pressure?
Could it be electrical? like the start of something like a coil failing? 
Would a new fuel filter do this if faulty? 
Thanks in advance K.

Check to see if all your in tank hoses are tight and not leaking. I've heard of fuel filters collapsing but mostly due to age, what brand filter?
Regards Martin.

Try pinching the front hose closed for 20 seconds or so and reving the engine. Sorted mine out and someone posted that it was probably a sticking non return valve in the tank. The increased pressure by pinching the return line closed cleared whatever it was.

Hi Guys thanks for your posts The fuel filter was a Luftfilter I got it from James Sherlock.Do you mean pinch the pipe inside the tank? or near the fuel rail on the outside left of the bike?

At the front tank connection return hose where it connects to the tank outside.

Thanks for that nobbylon,
 so an update I read in the hayns manual about re pressurising the fuel system - basically disconnection the pump when the engine is running and letting it run dry then re connecting the pump which i did this morning, Is this something i should have done after fitting the new fuel filter? anyway i drove it to work today I got about 6 miles it did the same again on normal temp in 5th gear as i de-accelerated for about 5 seconds the only thing i did when this happened was tap the start button and accelerate and it recovered i then did about 30 motorway miles and the same on the way back without it happening again? so i will see how it goes over the next few days as i have a lot of miles to do this weekend! K.

I believe Luftfilter is a good brand so extremely unlikely it has suffered a collapse. It sounds like the pump is cutting out, when you hit the starter when it cuts out you are resupplying power to the pump. Or possibly the infamous four pin connector to the pump.
Regards Martin..

MartinW wrote:I believe Luftfilter is a good brand so extremely unlikely it has suffered a collapse. It sounds like the pump is cutting out, when you hit the starter when it cuts out you are resupplying power to the pump. Or possibly the infamous four pin connector to the pump.
Regards Martin..

Is that the 4 pin connector outside of the tank behind the side panel on the right side?

That would be it, it can sometimes be cleaned (DeoxIT) and the pins slightly squished to make better contacts. The best solution with mine was to replace it with a waterproof 4 pin connector.
Regards Martin.