Ours has been under a cover for two years, but in the open. I try only to cover it when it's dry and give it a drive once a week or so. But some mould appeared on the inside of the canvas (Exmoor) last winter. I cleaned it off reasonably successfully, but am fully expecting it to mould again...

I had completely identical symptoms. It wasn't the carb. The points gap was way out of whack. Mark