IMHO it's better to either (a) soak it with something milder like Berryman B-12 or similar (no, acetone too harsh for plastic parts inside a B&S carb, incl. the pickup tube within), or (b) look for a revised replacement carb + tank online , either from eprey or similar.
The way I see it: (1) if the carb can handle gashol (E10) w/o sputtering or runability issues then all that's ever needed is just a throughout cleanout of the tank, a new carb base replacement gasket and a nice cleanup job on the carb. (2) if the carb itself is from the non-E10 era and over 25+yrs or older, it's better to get a revised one instead (granted that you are willing to spend that much to review an old B&S mower, choice is up to you).
Newer or revise carb is meant for handling broader based fuel including E10 which older ones may not be able to (unless your carb comes with adjustments or a means of re-jetting).
Good luck.
Q.
p.s. I dealt with a couple of B&S with a non-adjustable carbs and are all 30+yrs old, all with runnability issues (sputtering when under load) and ended up with revised carbs due to E10 that has been in-place in our are for over 20 yrs.. runs like a champ afterwards.
