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Just a thought, blocked exhaust will cause the same problem. Old double wrapped pipes would sometimes buckle on the inside and not be visible on the outside - try running w/o exhaust if you had one on for your street test



My first thought after reading this was Ruh Roe... 20 years ago we built a SBC for a truck. Took 3 months and mice had set up home in the exhaust... didn't know it and new engine had extended cranking and general "valve train" issues. Dropped the down pipes,,, started and ran good (for a SBC that is)
Curt

Oh yeah...THAT'S gonna leave a mark!