It sounds like you have an intake valve adjusted too tight. Retorquing the
head is a good idea but that will make the valves even tighter. Get some
feeler gauges and make sure the intakes are .007 and exhausts have at least
.014" of clearance between valve stem and rocker arm. It's all in the shop
manual. The tip off was backfiring during waqrm up, when valves tighten.
the push rods warm up slowest, so it takes them 15-20 min to reach their warm
length. You should also change points. That can work miracles!
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