The carb body and /or crankcase is distorting when you tighten the screws. It is a common problem with the 45.

The best fix is to lap the barrel to the housing while slowly taking up the screws until they are tight. The engine must be stripped completely to do this. You can use regular toothpaste as a lapping compound and carefully wash it out when finished.

The only other fix is replacement of one or both pieces.