When you want to understand something, before asking it is helpful to either search the forum or to use google. If you google the terms anti-syphon valve purpose, I'm sure you will get lots of hits. But basically, the anti-syphon valve is a spring loaded check valve that prevents fuel from being spilled in the bilges should the fuel line rupture between the fuel tank and the fuel pump. If the valve becomes stuck, the amount of fuel reaching the carbs will greatly diminish and even become zero.
When it comes to erratic speed on a dual carb engine, unbalanced carbs will do that. For all the cylinders to fire with the same power, all of them should be receiving the same amount of fuel/air mixture, but if the carbs are not balanced that will not happen. The purpose of the uni-syn tool is to verify that you have the same flow of air on each carb at the time. It is basically a manometer designed to measure vacuum.
If you have the Seloc manual, it explains how to use it. We have also touched this subject in the past, so you may want to search the forum.