No manometer is needed. Many, many people synchronize carbs with a length of tubing and listening by ear -- and adjust it accurately too.

5 PSI is good, unless the float bowl overflows -- you'll see it run out. The fuel pressure is not directly related to the jetting, because the fuel bowl is vented to atmosphere. Instead, the fuel bowl *level* sets the baseline. By contrast, with EFI the fuel pressure directly relates to rich/lean setting. So check the fuel bowl level - too high means too rich, and too low results in lean mixtures.

Weber will not warranty that their inlet valves will close at 5spi (but usually they will). Mikuni recommends 2.84 - 4.98 psi. Dellorto DHLA I've heard will run great at 2.5 to 4.4 psi.

A stock A-series pump is high volume and with the correct pressure for carburetors (2.8-3.8 psi). But the original 1970-1973 A12 fuel pump is spec'd at 2.6 psi, so check the pressure with a vacuum gauge. Later A12 is spec'd at 2.8 psi. Always test to ensure it is within specs.