Can you describe this a bit more? Did it cough, belch a bunch of smoke and quit or did it sag/hesitate and die? The first is a rich low speed needle and the latter is lean.

w8ye is most likely right about the engine needing more breaking in. Carb air leaks usually either make the engine difficult to kill with the throttle closed or cause the engine to run leaner, not richer.

After it's run in some more you can try a new OS A3 plug, too, which is a bit hotter, I think, than the Tower plug.

David