If it runs better with choke than without, there is little doubt it is running lean. There is a possibility that it has an intake valve leak which is puffing back through the carburetor and causing symptoms of lean mixture - you can check for this by running it without the air cleaner, and checking for a sort of puffing of fuel out of the carburetor's air intake. However it is rather more likely that the mixture really is lean, due to lack of fuel flow through the carburetor. If you can't make it rich by turning the mixture screw anticlockwise, then you almost certainly have a clogged carburetor and it needs to be cleaned out. The usual problem causing this outcome with your model is that it has accumulated debris in the fuel intake filter, which is a strainer on the bottom of the carburetor's fuel suction pipe. This is attached to the underside of the carburetor, hanging down into the fuel tank. There are two ways to access this strainer. You can undo the screws attaching the fuel tank to the carburetor, and lower the tank, leaving the fuel suction pipe projecting downward from the carburetor. The intake strainer is visible underneath the bottom of this pipe. You can clean the strainer in place, or on some models of Vacujet, the pipe is plastic and can be unscrewed with a spanner so you can clean it on the bench. The second way to access the filter, and the one I prefer, is to disconnect the speed control cable and then remove the carburetor and fuel tank as an assembly, by undoing the two screws attaching the carburetor to the cylinder's intake port. Carefully unhook the governor's wire link when you do this. Once the carburetor and fuel tank are on the bench, you can remove the tank, turn the carburetor upside down, and access the intake pipe more conveniently. You can also remove the mixture screw and clean the main jet.
