My 2000 Vulcan 800 Classic has gas in air filter

Hi and welcome to FixYa,

Your posted description indicates a carb overflow and that the carbs require a pulldown, disassembly; soaking in carb cleaner, cleaning the jets, vents, holes, ports and the walls of the bowl and inner body for the varnish like gunk of old/stale gasoline specially the fuel inlet valve.

As fuel is fed to the carbs,  the floats in the carb's bowl would rise up and engage the inlet cutoff valve closing the inlet port and therefore no further fuel would flow in. Fuel overflow would then indicate possibly that:

• the valves are not closing;
  
• their is an obstruction along the inlet ports and blocks closing of the valves;
  
• floats are stuck open or punctured;
  
• the float height would require adjustments;
  
• necking, ridges, grooves, scratches exist on either the valve or the port allowing fuel entry even if closed.
  

Mine was doing the same thing. It turned out to be a bad fuel petcock. If you pull the fuel line off the petcock while it is in the on or reserve position and fuel comes out of the petcock, it is bad. Besides coming from the air cleaner, your engine probably has a lot of fuel in the oil which will damage the engine if not changed.