Oh, bugger. It's idling over 1000 on the tacho, haven't checked in GuzziDiag. Might have to go over this again....
Have I invented a new procedure, BTW? 😉
Thanks for all your help.
In case anyone else is in despair and reads this, here are some things of note....
Things that were compounding the problem:
Jammed up white knob and sync rod thread would not allow easy adjustment to get rod on the ball joint without putting something else out of adjustment.
Fast idle had somehow crept in after setting idle screws, nudging the right throttle more open before attaching rod, make sure it doesn't nip up the throttle when tightening back up.
Possible exhaust leaks.
An ECU map that was richer than standard, making the left cylinder (which nearly closed) too rich at idle, therefore sooty.
Possible air leaks around perished rubber intake.
P.S. I saved myself £90 as well, which is how much a Guzzi specialist had quoted for a TPS reset, and I wouldn't have had the satisfaction of knowing I had done it myself, and knowing how everything was set.