[orbit398]

I have a 289 with the petronix ignition system. This morning, while the engine is at idle, it doesn't fire on all cylinders as it is running rough. Once I give the engine some gas, the non-firing cylinder picks up and the engine runs fine. I don't understand this. Suggestions? The engine has new spark plug wires and plugs.

 

[geegee]

Did you gap the plugs before installing them? .035 gap is correct for factory coils. A bit wider if you have Pertronix or Mallory coils.

 

[66stangFb]

While the intake manifold is still cold it is not uncommon for the motor to run rough. When cold the carb is not able to atomize the fuel, some raw fuel runs along the bottom of the manifold and the plugs load up. A blip or two of the throttle fixes things. Performance modifications (Carb, intake, heads, cam) tend to make it worse when cold.

 

[dailydriver]

New parts can sometime be defective, or fail in a short time. I'd first try to adjust the idle screws to get as smooth an idle as possible. Then, look at the cap, rotor, plugs, wires, kind of in that order. It'll be something simple when you find it. Good Luck, and keep us posted.