Broken Exhaust Valve Spring

[rbuckin111]

I recently installed a rebuilt 230 engine in my 64 GMC. The other day it was running perfectly and just quit. When I tried to start it up again, it ran very poorly like it was missing on about 4 cylinders. Upon investigation, I found the exhaust valve spring in the number 2 cylinder broken. Anyone have a thought on how one would break a valve spring? Should I be looking further for the cause of this spring breaking? As serious as this is, I would have thought this would have only affected one cylinder and that the engine should have run pretty well with only one miss. Have I only discovered the tip of the ice berg? Thanks for taking a minute to give me your input and ideas.