Yesterday I pulled the tank again and went over the vacuum tubes in detail - no breaks, no problems. After putting it all back together again I took it out and had the same hesitation on throttle and back firing. The bike feels seriously down on power too.
I was hoping to send the carbs down to Shoodaben Engineering in Florida for a spa session with Steve. His prices are more than fair, but after having an economist (whatever the hell that is) as a Prime Minister for eight years, Canada's dollar is in the toilet and my $500US carb repair would cost north of $800 with shipping, customs and the exchange rate. I paid $800Cdn for the bike in the first place.
So I'm rebuilding carbs!
In spite the many terrifying stories of carb removal on a Concours, I found the process pretty straight forward (thanks to Steve's video). Warm up the rubber on the airbox to carb, they get nice and soft, and you wiggle the whole thing free. With the carb on the bench, parts are ordered ($200Cdn for 4 kits - 1 for each carb) and I'm beginning to break it down to rebuild each.