OK..we now know the mixture is OK...and all the carb rubbers etc are OK ( btw they can still crack and be hard to spot even when fully tightened)?
Have you been fiddling with it to maybe cause this fault or did it just start to occur on its own?
Obviously both plugs are the correct ones ( simple theories first) ?
Maybe try emptying the float bowl of the 'weak' running carb and filling it from an aerosol with carburettor cleaner. Then turn over engine to see if any minor blockage is flushed away! Worth a try before you have to dismantle the carb etc.