Tuning the float level
"On Holleys this adjustment is very easy as it can be done without dismantling. With the engine idling take out the sight plug on the side of the fuel bowl. Be ready with a rag to catch the fuel that might run out if the float level is too high. With the sight plug removed, use a big screwdriver and a 5/8”/16mm wrench to adjust (the screw should just be loosened, adjustment is done on the nut). Adjust so that fuel is up to the level of the sight plug hole and just slowly trickles out. Note that if the level is too high and you adjust down, you have to wait until the fuel is consumed or trickled out, which can be an issue especially on the secondary fuel bowl. Also note that if you adjust down too low the nut loses its grip on the float bolt and when you try to get it back up it won’t work. In that situation, stop the engine, take off the screw and nut entirely and screw the float bolt up with your fingers. You may need a new gasket after doing this.
When both floats are adjusted as specified above, you can screw in the sight plugs."
On Holleys I take out the sight plugs even though they are supposed to be see-through, too difficult to see. On the Summit carb on my Longchamp the sight holes are bigger and with glass instead of plastic, gives perfect vision and no fuel spill needed.