Ok so faulty stick coils replaced and, although better the problem remained. So I booked the bike in for a diagnostic check with a local forum member and ex-mechanic who had the all of the electronic diagnostic kit.
Ahead of the drive up today I cleared a space in my own workshop on Thursday night and put the GS up on the bike lift so that I could set the valve clearances change the plugs and give it thorough once over. I set the valve clearances very carefully to spec and changed out the plugs the TPS, was adjusted to 37v as it was slightly outside of spec (42v).
After changing the the stick coils this week I noticed that the problem had moved up the rev range and I made me wonder if the problem was additionally fuel/air related. I checked all of the intakes for leaks and found nothing. I then syringed injector cleaner into the injectors and let it sit for an hour or so. I had run the tank low and filled it with shell V-power fuel mixed with injector cleaner.
I set off this morning with the bike still displaying the fault but after about 25 miles the bike seamed to clear it's throat and just started to run on song. By the time I arrived at Ian's workshop it was running faultlessly. After plugging it in and clearing a couple previous faults related to the TPS adjustment etc the bike showed up as clear. The throttle bodies were then balanced and after a complimentary coffee and biscuit I was on my way home.
So it would appear that aside from the stick coil I also had either:
1. a blocked injector/injectors
2. Water in the fuel tank.
When I went to fill it up yesterday after washing it ( not jet-wash) I noticed water sitting in the filler cap recess. The small drain hole appeared to be blocked and was not draining water. It's easy to see how this may have allowed water into the tank at some point.
Anyway continuing to keep an eye on things and I will be replacing the fuel filter as soon as a new one arrives.
At least I know that the bike has been almost fully serviced a little earlier than I had planned to service it and most importantly I can now start enjoying the GS as per the original plan
Many thanks for the input.
