Open the throttles when performing a compression test. You can't get a good compression reading with them closed.
Try testing them dry then inject a bit of oil in each cylinder to see how much the compression goes up. Big difference can mean ring wear but these engines are usually bulletproof. How many miles on the engine?
Just because all the settings are the same between carbs doesn't mean they really are. If the difference is at idle, adjust your idle mixture to make them the same. Not sure how that is done on the FCRs but it should be adjustable.
