Re: Primer bulb ?
When starting you need to squeeze the bulb until it is hard. This assures that the fuel bowl in the carb is full. When the engine is running it is normal for the bulb to be soft as the fuel is being sucked by the fuel pump into the engine.
Ditto and ditto to Bond-0's answer. With the engine off, the bulb pumps fuel by the fuel pump diaphragm and into the bowls of the carbs. As the bowls fill, the float closes off the float valve that allows fuel into the bowl. Once full, if functioning properly, the flow to the bowls stops and the fuel flow stops as there is no place for it to go. Hence the bulb gets hard because it pumps up the line and it all gets full.
When running the engine, the engine is consuming fuel from the carbs, hence bowls, so the float is lower and it allows fuel into the bowl via the float valve so there is flow. Flow means a soft bulb.
HTH,
Mark
