Holley Needle and Seat Repairs Pg. 1
Holley carburetors are sometimes known for flooding over in the event that a float sticks or sinks or debris contaminates the needle & seat, which prevents the needle & seat from fully sealing closed and shutting off the fuel flow when the float bowl is full. In severe instances this causes fuel to erupt directly from the float bowl vent tubes on the main body, but sometimes the problem is not as pronounced, which can make diagnosis a little more difficult. If the float sinks, the carb will require disassembly and the float will need to be replaced. However, more often than not the problem is simply a contaminated needle & seat, and the fix is merely a matter of removing the assembly, cleaning it of any obstructions, then reinstalling the assembly. Note that you do not necessarily need to remove the float bowls to perform these repairs, but it's not a bad idea to remove them since it makes it considerably easier to re-baseline your float settings. If you do not wish to remove the bowls, alternate directions will be given at the end of the last page.
To print a copy of this aticle or save a copy for reference please click on this link: Holley Needle and Seat PDF.
If your carburetor is flooding over but it is not coming from the bowl vents, the first thing to do is to check to see if there is fuel draining from the boosters with the engine off. The boosters are noted by the red dots in the picture to the right. If you see fuel draining from the boosters with the engine off, either the float level is too high, or there is debris in the needle & seat preventing it from closing.
Often times fuel will dribble from either end of either throttle shaft as the fuel drains from the booster and runs down the sides of the shaft. If you see fuel dribbling from these locations, chances are you've found your problem. Note that if you see a drip on the primary side, check the primary needle & seat, and likewise for a leak on the secondary side.
If you've removed the float bowl(s) from the carb to check for problems, the first thing to check is that the floats operate freely. They should pivot up & down with no sticking or "catch" spots. The float should pivot on its own merely by inverting the bowl.