This is a common one and I’ve addressed it before. A couple of people have had problems this week with twins running on one cylinder or misfiring on one and think they need to buy a new Boyer kit. This can not be a problem with the Boyer which fires both plugs together.
First thing – swap the plug leads and plugs over. If the problem stays on the same side the cause must be carburation or mechanical malfunctions on that cylinder. If the problem goes to the other side the cause must be faulty HT windings in the coil, faulty HT lead, plug cap of spark plug.