1969 Ford stumbling at initial acceleration
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.