AFP basics
Airflow Performance is one of the more common injection systems and utilizes a purge valve and associated fuel return line. The purge valve is mounted on the distribution spider on top of the engine. The purge valve is used to flush fuel vapor out of the system (up to the point of the distribution spider anyway), prior to starting the engine, especially for hot starts. It is also used to shut down the engine, in lieu of using the mixture knob. The purge valve requires a fuel return line that can either be plumbed directly to one of the tanks, or it can tee into one of the fuel supply lines running from a tank to the tank selector valve. For the latter case, when you are purging prior to a hot start, you would want to have the tank selector valve set to the opposite side from where the purge line tees in; otherwise, you would just be circulating hot fuel and vapor. So, if the purge line tees into the fuel line from the right tank, you would set the fuel tank selector valve for the left tank, at least for purging purposes.
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