For testing the needle & seat we used a Mighty Vac pressure tester. This tester will test vacuum, but also can be switched to apply pressure.
For most carburetors 5 lbs will be enough pressure, but some of the Holley 4 barrels will need to be tested at 7 lbs. Use any method of applying this pressure to the fuel inlet, but keep in mind that too much pressure will raise the needle allowing air to leak by. In the test we did in the video below we got the pressure up to 10 lbs + with no leaks. Test with the float in the float level position. This means the float will be resting on the needle, holding it in to seal. Applying pressure on the float against the needle may damage the viton tip, causing it to leak.
When the carburetor is off the car, remove the top to get access to the needle & seat. Apply some soap & water around the needle & seat, so that bubbles will form while testing when there is a leak.
A leak around the seat mounting may be just poor sealing of the gasket. Loosen and tighten the seat several time to seat it if it does leak. New needles may have residue on the viton tip, which may cause it to leak. Wipe it off with something like mineral spirits to remove the residue.
NOTE: A dry needle & seat might leak when a wet needle & seat will not. Needle & seats are always wet.
