The other thing to notice about this carburetor [and I am not trying to pick on these guys] is how black the venturi area and boosters are and the amount of build up. This comes over a lot of use but is usually the sign of an ignition or timing issue. Typically when we see that there is nowhere near enough base timing in the motor and the fuel is not being atomized properly as it leaves the booster.I might also add that it can be camshaft related and/or form an engine that is not sealing well. Base ignition timing can be based off the cam duration at .050 but in some cases if the cam is really big it may need to be locked out.The majority of the time when you have a blackening above the butterflies the fuel is being blown back from the cylinder and up through the carb whether it be from lack of base timing or reversion.