Not a lot to go wrong there….Gasket is in place under the seat? Float installed right side up? "Right side up" means that the cork is very close to carb housing when held upside down, and level, with about 20 degrees of arc. Upside down (incorrect) would be level with about a 1/2" showing between the float and carb, with 90+ degrees of arc movement. (I don’t even think you could put the bowl on this way; but stranger things have happened on this forum.) If the float is installed correctly, then sometimes there can be tiny debris that finds its way into even a clean carb. (See below) You would need to blow it out with some compressed air. Or, if you used aftermarket brand parts, they are often of poor quality and may not fit correctly together. Hold the carb upside down, bowl off, with a small piece of hose attached to the inlet hose nipple, and blow through it with your mouth. You should NOT be able to blow through. Hold carb upright and blow….float should swing down 20 degrees, the needle should follow the float arm and you SHOULD be able to blow through it. If it leaks in the upside down position, it is helpful to have a spray bottle with one third dish soap and two thirds water. Spray the mixture on the inverted carb, blow and look for bubbles.
Keep in mind, too, that 1957 is old. ( A GREAT year and motor; but OLD.) You should be using a pressure tank and dual line gas/air hose, from your tank. ALL of that stuff is also probably dirty inside, so you may just be pumping junk into your cleaned cab and causing the inlet needle to stick open, immediately….At the very least, make sure all is clean from the filter, on to to the carb…
Lastly, and I’ve only seen this a couple of times in 43 years, is that the carb body may be porous, especially if a really harsh cleaner or sand blaster was used to clean it. The soapy water trick will show that….although that is VERY unlikely.
Long live American manufacturing!
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