The plastic manifolds are prone to warping & leaking
The O rings need to be replaced as they flatten out allowing air to leak in.
In my shop they get changed without question as do all of the gaskets.
If it comes in the rebuild kit it gets used. No value in having a drawer full of 1/2 used kits & lots to loose having to do the job twice.
If possible I try to replace the plastic manifold with a metal one,
Not only does the metal one not warp it also gets hot so aids the vapourizaion of the fuel .
The carbs are designed to be cheap to make & fit not to run a long time with minimal servicing.
The first thing you do with a surging carb is to remove the blower housing so you get clean access to the engine
Start the engine, engage the blades to put a load on the engine then saturate the entire manifold area with WD40 from a trigger pack ( not a spray can )
While doing this you work the governor so the engine runs at high revs the let it drop back to normal revs to create a high manifold vacuum.
If the engine runs silly or you get clouds of white smoke from the WD 40 burning then you have a manifold leak.
You need to work the governor if you get no response initally because there is 5/8 of SFA manifold vacuum when the engine has no load running on high idle .
And don't overlook the butterfly shaft because they can & do wear, flog out oval and leak air.