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65 gto with 4 speed, rebuilt engine and carb, engine hesitates (misses) when downshifting and accelerating to lower gear. runs ok at steady speeds. has original 4 bbl carb, pretronix igniter replacing points. any ideas?
Depends. Is this a new problem or has the engine been doing this for a while? Was the rebuild stock or are there other aftermarket parts used besides the Pertronix Distributor? What things have changed on the engine since it last ran properly? Some things to think about are fuel related (octane, age quality). Perhaps timing affected by vacuum?
Could be a bad accelerator pump, bad plug wires, vacuum leak, ignition coil.
You say a miss, but also say hesitate, who rebuilt the carburetor?
Assume you have an AFB, has the rebuilder experience with them?
You can start with the coil measuring primary and secondary resistance or if you have a spare lying around replace it.
While your at it check the cap and rotor even if they are new and plug wire resistance. Try to eliminate ignition as the cause before going into carburetor issue.