I have a 1954 big bolt with a yf2100s carb. All the passage way are clear. I just bought the truck. It doesn't matter where the idle mixture screw is set, all the way in or completely removed. the engine runs the same. The previous owner "rebuilt it". When I removed it there was no gasket. It has 2 holes on the bottom surface. one under the mixer screw and one opposite of it. The one not under the mixer screw lines up with a groove in the manifold. I made a gasket with a groove for the hole under the mixer screw. The engine seems to run rich all of the time now. Before I made this change, the engine ran better when the brakes were applied. I know I have a hydrovac problem. I will get to that. What is the deal with the carb. The mixer screw had no affect before or after the gasket. Like I said before, as far as I can tell, all passages are open.