Not that I am an expert, but I can tell you about my most recent problem which
was very similar to yours.
Mine carbs have the vent tube on to of the float bowls and it had the wrong
gaskets on it. it needed the bottom gasket to have the three little cutouts on
it so the bowl can vent. My rear bowl was staying full and it drove me patzo
for over a year.
Hope this helps.
Suggested annual donation $11.47