Written by Dave "Doc" Langford
The square top carburetor has a vacuum operated accelerator pump system. If this does not work properly, the engine will sputter and cough when you try and accelerate it.
The accelerator pump is spring loaded. In other words it is always at the bottom of the fuel bore. When the engine is off the leather piston is at the bottom. When you start the truck, vacuum from the manifold pulls the piston up. This is why you need to choke a cold engine to help it start. The accelerator pump does not work until sufficient vacuum is applied. As you know when you accelerate the vacuum drops way off. If you still have vacuum wipers you know they stop when you hit the gas. So the theory is hit the gas-lose vacuum-spring forces down leather piston-fuel shoots into the engine.
Here's the problem: The rebuild kit that is sold is for a different carb (I don't know which one). They make modifications to the accelerator piston length and add a few new gaskets then sell it to you. Which is OK because you just can't find these kits anymore and all the other parts work fine. The spring in that kit seems too stiff and the kit’s acc pump shaft leather is also too hard and stiff. So two things occur, the vacuum cannot overcome the spring tension and the piston is too tight in the bore. I used an old piston and spring from another carb and the engine came to life. If you have an old carb remove the piston and soak the leather seal in any kind of oil for a day or so. Install it with the used spring and you should be happy. Note: if you need a carb rebuild on your square top, save the old acc pump spring and accelerator pump shaft with its leather seal to use with the new parts.
Also, you need to check the accelerator piston cylinder in the air horn for smooth action of the piston. Sometimes these carbs get oxidation built-up from setting for many years. If the piston does not move up or down without sticking slightly, then you need to take a brake cylinder hone and hone the cylinder until the piston can move freely. If the piston has any oxidation, it to needs to be cleaned with fine sand paper as well. Same for the piston/rod in the carb body, the cylinder has to be smooth and free of any roughness. Two items that are very important; the air/fuel passages must be open (not plugged) and the gaskets must be in good shape