Six months is not long enough to create varnish in the tank.
A fuel system consists of a tank to store the gas, a pickup tube to extract the gas from the tank, usually combined with a float to indicate how much fuel is in the tank, lines to convey the fuel from the tank to the engine, a pump to suck the fuel from the tank and push it into the carburetor, a filter to keep contaminants out of the engine and the carburetor, which feeds the fuel to the engine using a system of meters to feed the correct amount of fuel depending upon the demands of the engine for fuel.
Only two of these components can affect flow; the pump and the carburetor. The pump is mechanically driven by a lobe on the camshaft As the engine rotates, an arm is alternately depressed and released, causing a pumping action.
Unless the pump has been replaced with an electric one, the chances of it creating too much pressure (and thus flooding the engine by overwhelming the carburetor) are fairly low. If the fuel pump is electric, then that could very well be your problem. It could be over pressuring the fuel forcing the metering valve on the carburetor open despite the float.
When fuel is pumped to the carburetor, it enters through a valve that is opened and closed by a float. Gas is held in the main chamber(s) of the carb. As it is fed to the engine, the level drops and the float with it, and the valve opens to allow more gas to enter to the chamber(s).
The purpose of the chamber(s) is storage, nothing more. It holds enough gas to ensure that the engine can never be starved for fuel no matter how much demand it creates.
If the float is set incorrectly, gas can overflow and dump into the intake manifold, flooding the engine. And you will often see it coming up through the venturi tube, just as you describe.
From your description, it sounds as though the float is sticking so that the valve never closes. Take the top off the carb and inspect the float. Make sure that the pin that it rotates on doesn't have any burrs or isn't putting the float in a bind. Inspect the openings in the float that the pin goes through. Are they bent? Could they cause the float to hang?
WARNING: NEVER start the engine with the top off the carburetor. It is an extreme fire hazard!
Without gas in the carb, the float should move freely up and down without any feeling of restriction. With the carb assembled, you should be able to turn the carb upside down and then right side up and hear the float moving up and down to its stops.
Double check the float measurements. Carb kits usually have a chart that contains settings for multiple applications. Make sure you're using the measurements for the correct one. There should be two measurements - depth to empty and height to full.
The valve should have been a replacement part in the kit you bought. If it was not, check the valve for damage. Make sure that it will seal properly. You can do this by removing the float, manually holding the valve against its seat (don't put a lot of pressure on it) and blowing through the intake. If you can hear air, the valve needs to be replaced.
