Originally Posted by flyinga
To answer your question: yes. Constant flow with no purge valve.
Then it is common to see a bit of fuel in the air plenum, and from the sniffle.
The engine driven pump creates line pressure by pushing a diaphragm with a big spring. The engine's pump cam (via the pump pushrod and pump lever) compress the spring, then rotates out of the way, leaving the spring free to push the diaphragm.
Now shut down the engine using the mixture control. If the engine stops with the pushrod on the pump cam's base circle, the pump spring is free to push, and 25~30 psi remains applied to the servo inlet. The only thing preventing flow is the mixture disc valve....which has a leak rate. The rate is small, but it's there; it is not a zero flow valve. The leak sends fuel to the divider, onward to the nozzles, into the manifolds. Some of it runs down to the sump plenum. For the most part, the leak rate is offset by evaporation rate, and the difference is minimal.
The disc valve is common to the Bendix RSA and its variations, like the current Avstar controller, and late model Airflow Performance FM-150. Those of us using a "classic" Airflow Performance controller like the FM-200 have a drum type mixture valve. It has a much higher leak rate (1 to 3 lbs per hour) and would rapidly dump fuel into the manifolds in the above scenario. So, AFP units with drum valves also get a purge valve. When opened, the purge valve re-routes fuel from the control, before it reaches the divider, and sends it back to a tank. In consequence, purge valve users see less fuel in the manifolds after shutdown. It's limited to the very small amount which may boil out of the divider and spider lines. That happens with both mixture valve types, purge or no purge.
Anyway, the much reduced fuel dump is why purge valve users see less need for a sniffle.
BTW, a purge valve can
be installed with any fuel control. The reason for its existence is per the above, but the serendipitous feature most loved is the ability to flow a large quantity of cool fuel through the entire system prior to cranking.