Two trip-steps with these little 50s. The carb has a few little "secrets" associated with the emulsion tube/main jet. You should pull it, to inspect the main fuel passage, which tends to clog. When that occurs, no fuel flows through the main circuit and the engine won't take throttle. I believe that this model also requires a rubber seal, that gets sandwiched between the bottom of the float bowl and bottom of the main jet/emulsion tube. Without it, the mixture will be pig-rich.
As for the overboring, few shops have tools (boring bar, dingleberry hone) small enough to do a 39mm cylinder. If yours does, problem solved. These little cylinders are very thick and take an overbore like nothing.
I always verify the valve-to-seat sealing and contact band. Once in a while, I'll come across one of these heads that just needs a light valve lapping. Most of the time, a full valve job (new valves, seats cut, lapping) is needed. Was your intake port clean, silver, aluminum...or blackened. Anything other than squeaky clean means a tuliped (worn out & leaking) intake valve.