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When the motor starts crapping out (but way before it's ready to die) pinch the fuel line close and rev the motor once to clean it out and see if it idles ok for a while with the line pinched. Before the motor runs out of fuel release the fuel flow again. See if it wants to crap out, Repeat

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Brian, from the symptoms, it sounds like too much gas is getting dumped into the engine. In the long run, this is not a good thing for the health of the rings, nor the lubricating ability of the oil. Once this problem is resolved, do an oil change pronto.

What Allen is suggesting above has merit as an easy diagnostic tool. If you stop the incoming flow of fuel and the engine can clear out the excess gas and run well for a short time, you have confirmed that fuel is getting past one or both needle valve/seat pairs when it shouldn't be.

At the risk of stating the obvious, consider the function of the needle valves, seats, and floats. The job of the floats is to "float" (duh) on top of the fuel in the bowl. At a preset height, the float presses on the needle valve hard enough so that it "seats" in the seat (double duh) and stops the incoming flow of gas. My educated guess is that this is not happening in your carb. Some of the reasons that spring to mind are:

a. Crazy high fuel pump pressure.
b. Crap in the fuel wedging between the needle valve and seat.
c. Lousy/broken/worn out tip on the needle valve.
d. "Sunk" float. The soldered seams on the brass ones can give out. The phenolic/composite/plastic ones get can get porous over time.