I repeated the experiment and submerged the ends of the tubes in two cups. The 1/4" line still filled twice as fast. Then I closed the petcock with the 5/16" line and held it up in the air. I opened the cock and allowed the tube to fill slowly while lowering it to my bucket. The line then flowed the same as the 1/4" line, although the water exiting the tube looked like a barber shop pole. I submerged each line in a cup of water and they both filled at the same rate. After this, I could stop and start the bigger line and it would flow well. But if I picked up the line and blew back thru it to empty it, it would again flow slowly. It seems as though the inside of the tube must be fully wetted before it will flow full.
I agree that either size line will work with my .062" main jet, but I'm only producing 42 hp. With a single petcock with an 1/8" hole and 170 hp, you just might want to make sure you're fuel line is fully wetted before you start. Or get a larger petcock. A friend of mine was having trouble with his 1100 cc Suzuki which would run fine in the pits, but would lean out on the track. Turns out the petcock was partially blocked. Disconnect the line at the carb and hold it up in the air to make sure it fills. BTW, this is a good way to find out how much fuel you have left in a sealed fuel tank.
Tom