Has the fuel been sitting in an airtight container for a year? Is a year old and recently opened? was opened a year ago and has fittings that will allow air to get to it? If the fuel has been exposed to air at all for that time it will have a fair amount of water in it now. Maybe dosent affect the other engine noticably because it is broken in and well tuned.

When I got my .91AC the original owner stated he could never get it to idle. As I do with any used engine I did a tear down and found the cam gear was out by one tooth. Called Shadel and was told that the engine would run reasonably well that way except for idle. I re assembled it correctly and it ran and idled great.

You have one that runs correctly so I'm not going to suggest bad tuning as you obviously know what you are doing there.