4100 Flooding (too rich)
There are several things that make a 4100 4 barrel flood. Here is a list of things you can check, not in any order of importance.
Bad needle - Any pressure put on the viton tip will damage the needle.
Dirt in the needle & seat - Many good rebuilds go bad due to dirt in the gas tank. Even fuel filters can't keep it all out. Try blowing some air into the inlet to clear any dirt particles. If it keeps happening, then you probably need to clean the gas tank.
Test the float for any leaks. Heat up some water just prior to boiling and immerse the float. As the hot water heats the inside of the float, it will expand and blow air through any hole.
Check the float level. Set as per the instruction sheet. Do not fudge the float trying to correct richness. You are just hiding the real problem.
Be sure the clip that snaps onto the float pin and fits over the seat is there. A missing clip, or wrong clip will allow the float to float up via the pin allowing too much fuel into the carburetor.
Check the float to see if it is catching when going up and down.
Test the fuel pump pressure for 4 - 4.5 lbs. Add a regulator between the fuel pump and carburetor if necessary. New fuel pumps are especially suspect as they are not always made well.
The 4100 is susceptible to warpage which allows fuel to slop over the float bowl and into the carburetor bore. This can show up when going up and down hills. Use some flat stock to check for warpage. You can try a 2nd float bowl gasket. Do not use any sealer.
Look for extra gasket material under the seat. Fuel might be leaking past the threads.
A ruptured power valve can cause a rich condition. You can use a vacuum tester with some sort of attachment that fits one side of the power valve. You can also put one side or the other in your mouth and see if you can blow through it.
See if fuel is dribbling out of the main discharge at idle, or when 1st shutting the engine off. The discharge may be leaking. There is information here on how to test the discharge.
Black Smoke - too rich
Check your spark plugs for the correct burn color. Black could mean the jets are too big. Reduce by .002, white is lean and the jets should be increased by .002. Gray or tan is perfect. Leave the jets alone. This assumes the ignition is working properly. If the carburetor has been on the vehicle for some time and running OK previously, then your problem is not the jets. They don't all of a sudden change size.