Carbon build up on top of the pistons is normal, at least in all my experience it has been. I would look for any gouges on the cylinder wall. You would be able to feel them by touch. I would have the head checked by a pro. It cost me about $200 to have mine reworked. I would also install a jet valve elimunation kit. Those things are just trouble waiting to happen. Was there any build up in the piston that looked like antifreeze. Check your exhaust manifold also. These heads are bad to crack between the valves and leak into the piston and go right out the exhaust. Clean the bottom of the head with parts cleaner and check between the vavles for any possible cracks if you can't have it checked out. I would say that this is the perfect opportunity to have it checked out though and get a valve job. Taking that head of twice isn't fun and I only say that because I had to do it.