WRONG
all the mating surfaces need to be machined.
that's the metering block mounting surfaces, carby base, baseplate top and bottom, vacuum secondary actuator port and mounting area, accelerator pumps and the part on the bowl where they screw onto.
do NOT work on the metering blocks. the ridge around the edge of them is there for sealing of the gaskets. one of my metering blocks was slightly warped, so i had to change it.
i also converted the 4160 to a 4150 style carby by replacing the metering plate with a metering block incorporating 4 corner idle. i haven't done the rest of the mods for the 4 corner idle, so that will come later
machining the parts yourself is not a problem. all you need is a sheet of thick glass and some wet and dry sandpaper. and some time.....lots and lots of time. mine took about 30 hours, all up
here's the body after the machining
i started with 180 git and worked down to 400 grit to finish off.
just put the sandpaper on the glass, at the edge and hang the top of the carb over the edge of the glass. don't wrap the paper around the edge of the glass or you will machine a groove into the air cleaner mounting flange.
then, keeping even pressure and with some lube (i use degreaser) drag the body along the edge of the glass towards you. i use 10 towards me and then go to the other side and do 10 that way.
this method is a lot slower than using a file, but leaves less room for error.
just keep going until there's less than 0.005 clearance under a straight edge.
all the sealing edges and ports need to be as flat as possible. the accelerator pump port especially needs to be sealed from the power valve vacuum chamber.
for the base of the carby, i had to cut a piece of glass and some paper to fit between the metering block mounting flanges. from memory i think it was 140mm.
same story with the other bits. all must be flat to seal properly.
if you just replace gaskets and don't ensure the mating surfaces are level, the carby WILL leak either fuel or air. both are bad