That is the vacuum cut out switch…These engines have a tendency to “run away” when reduced quickly to idle in neutral. That switch is vacuum controlled and connected to one crankcase, so spark to that cylinder is cut out when excessive vacuum is sensed on that crankcase (high RPM with throttle valve closed). Needless to say, there are other switches involved that cancel this feature out when the throttle is set at higher settings. The engine will run fine without that feature being wired up, but it won’t have any “runaway protection”. You won’t be able to shut they engine off either. I sure hope you find the correct wiring harness for this engine!
