Car: 1967 Firebird Vert 326 Mechanic said what was happening was the seals were all deteriorating from not being run on leaded fuel.
Car will start and hold a rough idle but pretty much no power obviously.
Is it possible that the mechanic said the seats (not seals) were deteriorating? That would make a lot more sense, as leaded fuel kept the valve seats from receding. Once the seats are worn beyond a certain point, the valves can no longer seal the combustion chamber properly and the engine will start missing and losing power.
Did the mechanic do a compression test? It's not as definitive as a leak-down test but certainly will show if the seats are gone. Repairing the heads by installing hardened valve seat inserts will fix that problem, but as has been said, be aware that the engine may need more than that.
