Didn't by chance do a leakdown before tearing it apart did you? Which side is melted? Pics would help. Do a compression test? Could either been an air leak or plugged jets in carb melting it down. Clean the carbs, petcock, and flush tank before start-up. If they are stock carbs clean one at a time as they have different parts in them. If you didn't do a leakdown consider throwing head back on even if you have to use some sealer to rule out leaks at gaskets or crank seals, or case leak, JMHO.