At an engine rpm of 5,500 RPM the engine will need a carburetor capable of delivering 360 CFM assuming a volumetric efficiency of 0.80. I suspect you have more carburetion than you need. Have you pulled any of the spark plugs to see what they look like? If you have to much carburetion your plugs will be black and have a generous coating of soot on them. If the fuel to air mixture is correct the spark plugs should be light gray or very light gray; no black soot.
Some of the carbs that are used for a dual card setup can flow 360 CFM each. I think the old Holley 5200 carbs will flow more than 300 cfm (cubic feet per minute).
If you want really good performance you should carefullty consider fuel injection, either throttle body injection or port injection. I am in the process of identifying what I am going to use for my engine. Megasquirt or Microsquirt are two popular options that seem to work well on our air-cooled engines. It is my understanding that Microsquirt will handle both the ignition and the fuel injection. The ECU (engine control unit) will make your engine behave so much that you will enjoy your car much more. With fuel injection you will just make sure you have injectors capable of delivering the correct volume of fuel when it is needed. No more screwing around with jets and carburetors.
If the engine is running rich you may be able to smell fuel in the exhaust or exhaust pipes. The answer to your question is yes, you can use a single carburetor on your 2.1 Liter engine. You may want to put on a distributor with a vacuum diaphragm. You can purchase such distributors new, ready to install when the #1 cylinder is at TDC (top dead center). The engine operation on the street will be much better.
