James, just now seeing this, your experience is an almost exact replication of one I had back in October, also an Archer (III). Plane sat with full tanks in rainy weather for two days up at Addison. I sumped just before sunrise, both tanks, checked for color and smell, looked in both tanks, caps seemed tight, tanks full and blue, all seemed ok. Startup, taxi, run up, takeoff, climb to 2000, wait for further clearance, climb again to 5000, cruise for 45mins, fuel pump on, switch tanks, then watch in disbelief as fuel pressure went to almost zero and silence. I was over an undercast from 500-4000 with the ink still drying on my instrument ticket.
Mind you, I had set my Bo down in a field 3 months earlier, totaling it (engine grenaded, that’s a whole nuther story). “You’ve got to be ****ting me”. Switched tanks, waited, nothing, did some other pilot stuff trying to revive it. Declared. Just as I’m about to sink into the fluffies the engine started to sputter to life. Got a climb to 7000 and played around until I finally got it to run on the fuller tank. Landed and the engine sputtered on rollout.
Mechanic said they drained a full gallon plus from both tanks.