"Pressing lightly" on the accelerator pedal reminds me of a problem I ran into in the late 80s. We have become accustomed to, (or grown up with), electronic fuel injection. Engine Computers look at coolant and air temperature to calculate how much of a priming squirt to command from each fuel injector when we start the engine. In the days of carburetors, it was standard practice to pump the accelerator pedal a certain amount to create that priming pulse, and that came from the accelerator pump. Every car was different and developed its own "personality". Owners became familiar with the best starting procedures for their car.
When I worked for a Sears Auto Center in the '80s, one of my jobs was installing a really nice aftermarket cruise control system. I did that for a woman on the '80 Chevy Citation she had just bought. That was on a Friday. She returned on Monday, furious that I had damaged her car. She had to crank and crank to get it to start, but she admitted that once it was running, it ran fine. When the boss asked her to show him how she started the engine, her procedure did not include pressing the accelerator pedal. Turns out, it had been real warm during the summer, and like many cars of that era, the float was set so high in the float bowl, no additional priming was needed to get the engine started. Three days later the temperature had dropped to the 40s, and surprise, her lack of following the recommended procedure resulted in a crank / no-start. The boss flipped her sun visor down and showed her the starting instructions right there in front of her. She blamed the problem on me or the cruise control, but it could just as easily have been blamed on making too many right-hand turns, driving too long after dark, hitting a pot hole, or buying gas at the wrong gas station.
You may be running into something similar. A weak or dead accelerator pump will not give the needed amount of fuel for a priming pulse, and it will cause the stumble or hesitation when accelerating. When you open the throttle blade, air flow increases instantly because it's real light. Fuel is much heavier, so it takes some time to build up momentum and get flowing. That's the cause of the lean stumble. The accelerator pump pushes some fuel into the air stream to overcome the delay.
If you installed a used carburetor that has been sitting on a shelf for a while, it's a real good bet the rubber diaphragm in the accelerator pump has dried out and has become ineffective.
Sunday, November 6th, 2016 AT 7:12 PM