Could be carb adjust. Is it just a little cooler or cold? Is it an obvious miss on that cylinder or does it seem to run good? Idling does it occasionally pop in the exhaust? If it does a lot of times it is too lean in that cylinder. Carbs, ignition, compression, valve adjust can all affect how well a cylinder is firing. Checked how the spark plug looks? Check over the basics first, if it has been sitting for a long time the carbs are suspect in being gummed up and need a good clean and sync.
If you rev it up a little bit and hold it there for like 20 seconds, does it start to get hot? Having it almost cold at idle means it's not firing on that cylinder at that rpm. It could be a dirty carb or needing a carb sync.