The Cb550 carbs (not the PD types) are suspended by 8 rubber couplers. Four go towards the head and attach to the intake manifold. Four go toward the rear where the air cleaner is.
The rubber couplers toward the intake manifold are P/N 16211-323-000. When these crack/leak the mixture provided by the carbs is greatly effected.
The numbers Jerry gave you are for the metal intake manifolds themselves. If you have removed them, they have 30.8mm O rings that ought to be replaced (91303-001-020).
The rear rubber bits are attached to the "Air cleaner chamber Complete" (17310-323-000B or 17310-323-010B). The actual rubber bits were not available separately from Honda.
From reading this forum, I heard that David Silver Spares may sell the rear rubber bits separately. There was a thread by Jonesy, I think. Call DSS, PM Jonesy, or search the forum.
Small leaks in the inlet side do not drastically effect the carb mix, unless they are really large cracks. However, they do bypass the filter and let dirt into the cylinders. These can be patched with black RTV, if you can't find suitable replacements.
These rear rubbers are what give the compliance needed to extract and install the carb bank. Remove the band clamps to help them survive and loosen the air filter box mounts to allow the chamber to move back to the frame tubes. Every, 1/4 inch of space (and a spray of silicon lube) helps the couplers survive without tearing during extraction and installation.
Cheers,