[tsubaki]

I'd clean the carbs.
Rebuilding them may not be necessary.
Once in a while, certain carbs on certain motors got to certain locations. Mark or remember them as to where they came from, easiest thing is to do one at the time.
Clean the bowl thoroughly (carburetor cleaner in a spray can usually is fine), find a pick of some sort the same size as the jets you have, to ensure the jet holes are completely free of encrusted trash (a piece of wire, acetylene torch cleaning tip tool, correct size drill bit).
Normally it is the locations about 1/4" below the bowl gasket or float (locations where the gas just sits) that will have residue.