My boy tells me he went out to start his bike this weekend after the winter lay up. Bike only fired on the rear cylinder. It was running well when he put it in the shed. Now I plan to look at it today. Firstly I'll check for a spark then try and change the coils round before anything else but owt else I should be looking at

Next obvious one would be fuel, could be a stuck float on the carb, or blocked jet.

Yea, easy fix...dodgy plug replaced and away it went......