Does the 1926 gas tank have a vent? Cap doesn't seem to have one.
Is your cap made of pot metal? The original 26-27 caps were not brass or aluminum. That's one way to tell. I think my caps are vented, but I know they are pot metal.
Brief answer, Yes.
Allan from down under.
The tank is not vented. The vent is in the gas cap. If your cap is not vented the car will run for a while until it builds up enough negative pressure in the tank to overcome the slight drop from the tank to the Carb. Then the car will starve for fuel and stop running.
This earlier post shows pics of '26-'27 gas cap
Just drill a small hole in it and the problem is solved.
There are two vent holes in the 25-27 gas cap. One in the body of the cap, and another in the lower baffle plate.
The first picture below shows the gas cap from my pickup with an oval tank.
It's the radiator cap from some english truck from the 40's, I think, thus the "enfo" skript. Since it was a radiator cap it didn't have any hole and I learned during the maiden trip that fuel flow seemed to be erratical. Soon enough I drilled a little hole and the car ran much better without the need for opening up the carb needle periodically I had experienced before. When I drilled the hole I also made a cupped aluminum disc to fit in the bottom recess as a baffle. I cupped it with a ball pen hammer and filed it to fit so it could be fastened like a frost plug - with a hammer blow to make it expand. Some epoxi secures it extra. The holes aren't in a straight line to reduce the risk for splash.
My 27 runabout has a repro cap shown in the last two pictures. It came with a hole - but no baffle.
Should be possible to make one - though not as easy as in the brass cap, but I haven't seen the need yet.