Appologies for float it was the wrong word to use.
Mechanical fuel pumps
Impulse fuel pumps
Gravity feed
all supply fuel to the seat at different pressures.
The pressure that the float valve needle exerts upon the float valve seat might not be enough to fully close off the fuel supply when recieving fuel at a higher pressure.
And to obverse, too much seat pressure can cut off the supply too soon resulting in a fuel level too low causing the engine to run lean and even hunt a bit.
I have had this problem a couple of times and it took a looooooooog time to sort out because the engine was fitted with one of the carbs shown as being correct for the engine.
Not being a Briggs expert I always though they were all set up the same, just from different suppliers.
However with the Craftsman the carb fitted was but not for a fuel pump so the thing would randomly run rich, eight stroke a bit then carry on as normal, particularly when the govener allowed the butterfly to go WFO.
But the wrechard thing would not replicate this in the shop yet would regularly do it in the grass
Unbeknown to me the PO had gone to breaker and asked for a carb for a 13 Hp engine because his one had filled with water & corroded beyond use.
In frustrated desperation I changed the carb and the mower ran perfect.
Used the old carb on another engine with gravity feed & it ran perfectly .
Being used to Amal carbs that have random float valve seat heights I made up some float bowls with a nipple in the bottom so I can check the fuel height inside the carbs when I get one in that seems correct but just won't run properly.
It appears Walbros are not much better than the old concentrics.
Where as the Honda carbs are all exactly the same ( well as good as I can measure ) and the makunis fitted to the Vanguards as similarly very consistant