Well took my XS400/Sidecar for it's first short test run, about one mile round trip. It started with just a few kick and seemed to run pretty good, there was a little popping in the carbs when backing off. Then as I slowed down to pull in the driveway it stalled. Tried kicking it over and it would not start. Pushed into the garage and hooked up a spark plug to the right wire and kicked over and it seemed to have OK spark. To be sure it had enough fuel filled till gas was over the center hump. Kicked some more not a sign of firing. Pulled the drain plug out of the left carb and good flow of fuel. There is an inline fuel filter in the line from the tank. Pushed down driveway and tried to bump start in second gear, did this twice, and it seemed to fire some but not enough to take off running.
I was hot and tired so decided enough for today.
Might try pulling air filters and shoot some car cleaner in there and see if that will get it to fire. Did pull one plug and it was dry so don't think it was flooding while trying to start. If the carb cleaner does get it to fire up I'm going to spray some around the boots to the head, they looked good when I had the carbs off, nice a flexible and no obvious cracks.
EDIT:
Was doing some reading on carb problems and saw reference to "Butterfly seals" are there seals around the shafts for the butterflies in these carbs? When I cleaned the carbs I did not remove the shafts, I assume the shafts have to come out to replace the seals.
Despite this set back can say that it seemed to pull the sidecar pretty good, saw an easy indicated 60 mph on the level road.
Just need to get this thing to start as easy as the old XS650 I rode for several years, one or two kicks 90% of the time in all kinds of weather