This is a direct quote taken from a 1970's Carter carburetor technical publication:
Brass Floats vs Nitro-Phyl
In many applications the old reliable brass float has been replaced with the nitro-phyl design. Although it is more expensive, it will withstand the higher operating temperatures. These higher operating temperatures cause expansion and contraction of the upper portion of the brass float which results in cracking.
The old method of checking brass floats does not apply to the nitro-phyl design. The nitro-phyl float is checked by weight --- the 21-252 float used in all Thermoquad carburetors should weigh 7.5 to 8 grams.