my 1987 k100 , recently restored after a 15year layup, has been slow to start ever since the rebuild. It takes a 5 second push on the starter in all conditions, before it fires. At tickover, the engine speed surges and occasionally coughs.This symptom persists up to about 2000 revs, and makes the bike a pain to drive at slow speeds. Over 2000revs the engine is smooth and pulls like a train.. I recently renewed a hoseclip on the pipe from fuel pump to filter (inside the tank) and noticed that there was no residual fuel pressure, and as soon as the clamp on the pipe was loosened, air bubbles came out. I suspect the fuel pressure regulator, as it's the original, but it doesnt seem to leak when sucked on the vac hose. If this were faulty, would it allow air into the fuel system ?, if so, from where ? . Also, there are no obvious liquid fuel leaks, and no fuel smells.....all answers will be much appreciated.

Your starting and low speed problem, sounds like a intake leak.
About residual fuel pressure, it could be that your FPR leaking via the return line.