Found my problem. Balancer bolt was tight, but not tight enough. It was allowing the balancer to rock back and forth at low rpms. I called myself getting it really tight, but I didn't use a torque wrench. That carelessness could have caused me bigger problems. I will be torquing it properly this time. Thanks to everyone that offered suggestions.
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73 firebird 455 6x-4 heads 350 turbo 3.73 gears
1/8 7.65 @ 90.95
1/4 11.96 @ 112.60
new best in 1/8th 7.57@91 mph(4-13-12)
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