I expect it was an idle jet as well ..main jets are screwed into the float bowl I believe ....Because the orings get bad between the stand pipe and the bowl and pull extra fuel up and flood out.
..Probably can install complete carbs of same size and jetting but will need to do as pairs..Probably just find a dead deere for parts.
The outer air screws should be 1.1/2 turns out and might need to be glued to stop turning,,,,do not use air screws at any time to adj idle speed ...
There is also a power jet in the bowl ..
COLLECTING HYDRAULIC DRIVE YAMAHAS
EW643 , TL433 , TW433 , PR440
1972 TO 1977