It is possible for it to be fuel. The only way it could be fuel though, would be for fuel to be coming out of the petcock and into the float bowl of at least 1 carb. This could be due to a stuck float needle or the petcock just dumping fuel. From there it would have to over fill the carb to the point that fuel was seeping into a cylinder, out of the same cylinder and into the exhaust. This can happen if both intake and exhaust valves are open and the piston has a good ring seal so no fuel is getting into the crankcase.
You might have a fuel smell to your oil, too. Pull the fill cap and check. If you do then fuel is getting in there. This whole scenario doesn't happen often but I have seen it at least once.