The single most frequent screw up is the float level adjustment.
Most people just don't understand how Holley float level is adjusted, so they screw it up badly...
There are three types of Holley float bowls, but we are only going to cover the most common two,
Side float pivot, with the needle and seat on the side of the float bowl....
And Center pivot, with the needle and seat in the center of the float bowl.
Side pivot first...
IF you DON'T have two external fuel lines going into the front and back float bowls, you probably have the side pivot float bowls.
If yo DON'T have a Screw and Nut arrangement in the top center of the very front edge of the float bowl, you probably have a side pivot float bowl.
See graphic for the side pivot float bowl.
Now, you have a brass 'Screw' in the side of your float bowl somewhere.
(Usually on the side away from the throttle cable linkage)
You will have to take that screw out while the vehicle is running...
A word of caution here, You WILL have gasoline all over the top of the engine,
You WILL have a potential fire hazard.
You WILL need to do this outside where a fire will not spread to anything else.
You WILL need a fire extinguisher.
You WILL need to make sure your ignition system isn't leaking voltage and sparking.
You WILL need to have rags under the carb to catch leaking fuel from the site plug hole and the needle and seat.
You WILL need to take appropriate steps to avoid a fire and potential damage to you and the vehicle.
Take the sight plug (brass screw in the side of the float bowl) out.
Start the engine.
Adjust the needle and seat until the fuel barely creeps into the threads of the sight plug hole.
If you do heavy off roading, you may want to turn the adjuster nut in (lower the fuel level) one flat on the nut.
( 1 Flat on adjuster nut = appx. 1/32" to 1/16")
Loosen the LOCK SCREW on the needle and seat, and turn the ADJUSTER NUT to adjust the fuel level.
(This is backwards from almost everything else you will encounter.)
Turn the NUT right (Clockwise to LOWER the fuel level).
Turn the NUT left (Counter- Clockwise to RAISE the fuel level).
When you have the correct adjustment, hold the NUT in place while you tighten the SCREW down.
Replace the sight plug in the side of the fuel bowl.
Repeat with the back bowl if you have a 4 barrel carb.
Clean up the spilled gasoline.
Have a beer, you have just taken care of the #1 problem with most Holley carbs...
Just as a side note, these side pivot float bowls with round floats are the thing to have for low speed, off camber operation.
The long rectangular floats on the center pivot bowls have a tendency to open the needle and seat when the fuel climbs the side of the bowl in off camber situations.
Using the spring loaded 'Off-Road' needle and seat will help quite a bit also, as will lowering the fuel level one or two flats on the adjustor nut.
(Never more than two flats or you run the risk of uncovering the primary metering jets and leaning the vehicle out)
So many cats, so few recipes...