other tig welding techniques that work for a root pass
Another technique that works is using a 1/8" (3.2mm) gap and 3/32" (2.4mm) rod. With this technique the edges are melted to form what looks like a keyhole. Then the 3/32" rod is dipped in repeatedly. Some slight side to side torch motion is often used but if the fit is right, a root pass can be welded with no sideways torch motion.
Another technique is to feed the rod from the inside of the pipe. In order to make this work, a large gap, like maybe 5/32" gap while using a 1/8" rod. This method overcomes gravity on the bottom of the weld and is used mostly when a pipe joint is on jackstands and there is nothing in the way...but it is a hard method to learn and its really touchy when there are other pipes or other stuff blocking your vision or elbow room.