runs mint to 3800, then falls on it's face

[Rev]

The secondary spring is not the problem. What kind of fuel pump are you running? You should have 4-7 PSI (minimum) going to the carb. Check that first. Could be a weak fuel pump or a clogged filter or a crimped line. All those would show up as low fuel pressure. Get a vacumm guage at AutoZone or somewhere. Those have a pressure reading as well as vacuum. That fuel pressue reading should tell you something.

Rev

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'66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph
1/4 mi.