1976 Volkswagen Beetle new Beetle engine floods while drivi

I had a brand new engine installed into my 1976 beetle. I am trying to put a few hundred miles on it so the mechanic can adjust the valves and carborator before I put it away for the winter. But as I drive the car has trouble starting back up. When I go 20 miles it has a tough time, but when I went 40 miles the engine was flooded, would start, and had to have the gas drained (hydraulic lock). What would cause the engine to flood while driving?