Sounds familiar. A few short jaunts up and down the roads around camp, a lot of idling, then 12 miles up the mountain (4000’ to 7200’), and I had to switch to reserve a couple miles into our return trip. That’s where I learned that the petcock doesn’t want to let fuel past in reserve, and actually had to pull the tank from the 300 and drain some gas into the 350 tank.
Have you checked for blockage in the petcock or the reserve line. Get a beer line brush or s stout pipe cleaner and ream the line
Pulled the petcock and blew ~5psi compressed air into each tank line. Probably not an OSHA approved test, but I got bubbles from both lines. I think petcock is worn. Blowing air into it, no blockages in either position, but I still get some flow in “off” position and can feel air coming through the “on” when it’s in “reserve” position.
On the trail, I switched to reserve and the fuel pump started racing, but engine kept sputtering. I added fuel, switched back to main, and it was fine again.
I’m thinking the petcock rubber is shot, causing it to pull air when in reserve.