Here are the basics of how a how a poppet valve fuel mixer works. When the piston goes up, a vacuum is created in the crankcase. Air comes in under the poppet valve lifting it. Fuel is pulled up from the little hole under the poppet valve. It mixes with the air as it passes over the hole and goes into the crankcase. Pressure is created when the piston starts down closing the valve and pushing fuel into the cylinder for combustion. The amount of fuel that comes out of the hole is controlled by the mixture screw you’re holding in your picture. The purpose of the spring in your picture is as stated by Richard. It is necessary for the fuel mixer you have. That’s a poor substitute for the original.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.