Chemical cleaners are sold, but bear in mind that these products damage and degrade gaskets, O-rings and plastics. That is why I recommend that you disassemble it intact, using different containers so that they do not mix the parts of the carburetors. You can help the product by giving a brush, the most important is that the product enters through all the ducts, when you see that the dirt has loosened it rinsed it well with hot water to remove all the chemical, finally you do a complete washing of all the metal parts with naphtha or thinner (I use the latter) and then blow through with air under pressure, especially the air entering each duct. For the assembly place new joints, diaphragms and punzuar.