I've been going over the Shadow as you do with a new bike and noticed a misfiring on tickover and popping on the overrun, which I (and the salesman) thought were due to the previous immature owner having drilled holes in the end of the silencers to make it louder. I've had the plugs out and found it to be running weak on both cylinders but weaker on the front pot.
I took off the seats and tank etc. to check hoses for holes etc. and thought I'd balance the carbs at the same time with my new Carbtune balancer, which I did. I noticed though when blipping the throttle that only one slide is moving, indicating a perished diaphragm. I Thought I'd cracked the problem until I noticed it was the carb. for the rear cylinder!
The people I bought it from (Metric Cruisers) have said to take it in and they'll have a look and sort it, as they know the bike is basically sound having taken it in part-exchange and resold it a number of times.
I think it needs carbs cleaning out and new diaphragms fitting.
I've just got myself a compression tester from Machine Mart to make sure it's nothing more serious, like valves.