When you have a vacuum leak, you're drawing air in from the outside. When you spray a combustible liquid on that vacuum leak, it will get sucked in too. This changes the air/fuel ratio of the engine. So depending on what the air/fuel ratio is, the engine RPM will change (either increasing or decreasing). Most of the time, it increases since idle mixtures are generally failure lean. In this case, the RPM went down, as apparently the carb is jetted fairly rich and the starting fluid richened up the mixture past best power, to the point where the engine starts to lose power with more fuel. This makes sense in a race application, and @Half Fast had the carb jetted by a pro who knew how to jet the carb for racing (the jetting is the same as I haven't changed it, I just cleaned everything up).