Purge valve position while parked
 

Running mogas occasionally with AFP injection and a purge valve.

It is my impressiont that mogas varnishes much more quickly than 100LL and consequently, I am interested in the purge valve position that most thoroughly drains the splitter, spider lines, and injectors so that the fuel cannot degrade there and create deposits.

I have tried it both ways, both open and closed after shut down, without any discernable difference in the sounds, smells etc. that eminate from the engine.

Maybe Don has already weighed in on this?

The purge valve does not actually drain the system, it does however open a path for fuel to be rerouted back to the tank or wherever you have the return plumbed in.
Once the engine is shut down, either by pulling the mixture or the purge valve you can leave either one in the on or off position (preferably off).
The real reason you have a purge valve is to evacuate vapor and hot fuel and circulate fresh cool fuel into the system for an easy hot start.
Your concern about mogas deteriorating and gumming up your system is probably unfounded unless you are having your airplane sit around for many weeks at the time.
My solution is to keep avgas in one tank and flush out the system by running avgas for landing, taxi and take off.

Purge valve position while parked
 

On the ground. Purge valve in the off position (out) mixture control full rich.

As stated above the concern is the auto fuel varnishing things up when it evaporates. To solve this issue you need to flush the system with avgas before shut down. Switching the fuel to avgas before entering the pattern to land flushes out the system nicely.

Don

Purge valve position
 

Thanks Don!