The Holley 1940 carburetor originally used a Nitropyl type of float and because they tend to absorb fuel over time, we recommend that they be replaced at each carburetor rebuild. Once a Nitrophyl float has absorbed fuel it will become too heavy and allow more fuel to enter the bowl than what is needed, causing a flooding effect.
The only way to test the Nitrohyl float is to weigh it. The 1940 float should be 12.0 grams. If you don’t have a scale, then replace it.
The Nitrophyl float can be identified by the black color and it looks like it is made of a plastic material.
The Nitrophyl float has been replaced by a brass type. The brass float is tested by heating up a pan of water just prior to boiling, immerse the float, look for any bubbles.
This carburetor is one of the hardest carburetors to identify when it comes to purchasing rebuild kits. Fortunately there are ways to find the original carburetor number, or worst case and with a little work, you can at least get the correct carburetor kit.
Identification. Rochester used a tag attached to one of the float bowl screws.
Some of the B type carburetors also had the number stamped on the face of the base (flange).
Some Rochester B carburetor were produced without tags. On these carburetors the last 4 digits were stamped on the bottom of the throttle body. The carburetor number was also stamped on part of the air horn gasket, but the chances of the original gasket being found is pretty low. Add 700 to the 4 digits.
When all else fails you can figure out which carburetor kit to buy by inspecting the accelerator pump. The older Rochester B carburetor used a flat stem pump and and is found here. The later Rochester B carburetor used a round stem pump and is found here.