When you sayd "lots of gas to the carb" do you mean that gas comes out from the fuel line when you unscrew it from the carb? If so, the fuel still has to go through the viton float valve/needle area. I had a similar problem a few weeks ago. My carb wouldn"t stop the flow of gas going to the carb, and after running for 30 seconds or so it would get sluggish and black smoke would come out the exhaust. I would quickly shut off the fuel supply at the sediment bowl, and the engine would run normally (until it ran out of gas in the carb bowl). I ended up putting in a new float, and a new float valve/needle/gasket. It solved the problem for me.
Oh, could you possibly have water in your gas? If you have ethanol blended gas, the water can build up in your fuel system.
Also, check to make sure your air filter is clean. Try running it briefly with no filter.
Another problem I had is when I put on a new gas tank cap that I bought from Autozone. It seals the hole real good. Really really good. So good that it is airtight and the engine won"t run after a few minutes because of the vaccuum in the gas tank. I have to loosen the gas tank cap so that it isn"t air tight.
You might also want to check the condition of the distributor/rotor.