After a long time in storage working on bringing a 1983 Suzuki GS550 back to life. Carbs have been fully cleaned, a few times now.

Bike starts but as soon as it heats up climbs in revs and sits about 5k. Cylinder 4 seems to be cold and not firing. It has two carbs feeding 4 cylinders and the other cylinder from this carb seems to be getting fuel and firing. Similarly it has two coil packs and the other cylinder with the same coil pack is firing.

When out of the cylinder plug appears to be sparking. All other plugs have slight purplish coloration on electrode which I assume is causing by spark and not cylinder temp. Is it possible that a coil could still give enough to cause a spark but break down as it heats up in operation? I’m at my wit’s end with it. Any suggestions?