Nope, it's not a float problem. That would show up at high RPM's first or going around a sharp corner. If it runs better with the choke on, it's running too lean. That could be a vacuum leak or a partially plugged idle fuel circuit. I've also seen faulty gaskets that blocked (or opened) a circuit causing a flow problem too but it's pretty rare and I haven't seen that with a 1920. Most of their problems (other than the chronic fuel bowl cover leaks) are in the metering block. You say you have no vacuum leaks.....have you checked what the manifold vacuum reading is?