OK, after replacing the fuel I did a test ride today and the sputter is not fixed.
Symptoms: The bike will start fine. It will run OK for about 30 seconds after which there is a gurgling (underwater sound) sputter / misfire that occurs first at 2000 RPM and then seconds later at 1500 RPM and finally the bike if left to idle will die after about 3 seconds. This does not require load - is repeatable in neutral. Bike will start and show same symptoms (idles fine at 1st etc.) after 5 minutes rest.
Bike history: Always been a strong runner, has approx 60K miles and is ridden at least every 2 weeks. Bike has always had a weak elec system, I have replaced regulator 3-4X over 20 year period. Bike suffers from a parasitic elec leak, I have to put it on charger before a ride if it has not been ridden the day before. This has resulted in long periods of the bike being on a charger. Bike has always had a rough spot at 2800 RPM - never could fix it. The 2800 stutter is exacerbated by hot weather but I can typically throttle "thru it." Minor note: Temp shielding on fuel lines to avoid fuel boil has had no effect on temp stutter.
What I have checked / recently touched:
a. Bike exhibited the sputter for 1st time week ago when Reserve Light was indicating low fuel level. This was at 60 MPH / 4K RPM.
b. Adding fuel allowed bike to get home fine (approx 3 miles).
c. 3 days ago fuel tank drained*, new fuel (from working Honda bike) added. I was careful to get fuel into both lobes of tank - me not being sure which side sucks the fuel. Still, the fuel tank is not full. I would estimate there is only 3/4 of a gallon in the bike.
d. *When tank was drained it was removed. Upon installation of the tank I turned the rod on right side all the way counterclockwise (see pic). The knob on the left is fully turned clockwise. ***Is this correct?*** Elec connectors were mated under tank as per usual.
e. Bike started fine after fuel tank installation w/ pint of fuel. Fuel reserve light was not lit (but it is unreliable).
f. With 3 addit. quarts of fuel added, a test drive --in warm but not hot hot weather --showed symptoms above. This stutter is far worse than the temperature stutter noted in history section. LED showing battery level showed the battery was near fully charged during test.
g. 6 months ago I replaced main battery cable with new, super conducting 3rd party cable. Prob has 500 miles on it - no issues. Fuse on new cable is intact.
h. Main Guzzi fuse box today revealed a burnt 30 amp fuse. I replaced the fuse but symptoms continued. A 20 amp fuse was replaced with the correct 15 amp fuse. No effect.
i. One of the battery terminal screws was not fully tight. When tightened, symptoms remained.
j. Spark plugs changed 100 miles ago. Spark plug wires have not been replaced for over a dozen years.
k. No change in exhaust smoke noted (exhaust not visible).
<pics below>
Thanks for your advice
Bob
p.s. Air filters removed for pix