1969 Ford stumbling at initial acceleration

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1969 Ford Mustang Grande with a 351W, Holley 3160 vaccum secondaries, headers - otherwise stock motor.
The carbuerator was just rebuilt, accelerator pump is new, power valve is new, timing is correct along with dwell. Good strong fuel squirt when throttle is opened.
Here is the problem: On initial acceleration under load (in other words when driving the car normally), the car stumbles. If you 'feather' the gas pedal, it will lean backfire sometimes. It will even stall occaisionally. After the initial stumble, the engine runs fine.
This problem is barely noticiable under acceleration in park. There is a slight stumble, but nothing to write home about.
Smooth acceleration in park shows no stumble. Very gentle acceleration under load shows no stumble.
If the throttle is opened wide and hard during normal driving, say like passing, there is no stumble and the engine runs strong.
Any thoughts?????

posted by  jhs130

Pull a spark plug from one of the rear cylinders - 7 or 8 - and check if they have a lot of carbon on them. I had this happen before and the carbon on the electrodes would cause the same symptoms.

posted by  67Coronet383

Those symptoms sounded like the same with my car, just pulled #8, and it was loaded with carbon, they are really old plugs anyway so I'm going to pick up a new set.

thanks 67Coronet383.:thumbs:

posted by  azkid110

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