I spent the day with my daughter going through this motor. It was built on the cheap for a guy who wanted it that way. Rods an pistons matched somewhat . Heavy rod with light piston etc. I don't do this anymore even though it worked in the past. In this case I am guessing the crank is way out of balance as well as discrepancy in piston and rod weight. Valve train is good so will keep that. I have a set of reconditioned rods, a fresh ground 020/020 crank as well as new Egge pistons and rings. Will take block in for a rebore. New rotating assembly will be professionaly balanced.