Advice on 6G TIG carbon bead

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Hi guys, I've been welding for the last 4 years, so im a capable welder, but I have mostly been welding only alloys for the last three.  I keyhole my beads and walk the cup on all alloys which always produces great results, but on my carbon Im having a hard time pushing the bead in (on the bottom)past flush whether i keyhole it or walk over the wire.  The top half looks ok... It has been a while since i have welded carbon, so i would appreciate any techniques, gap, rod size, and tungsten angle advice you might have.  I have to take a 6" sch40 6g test soon and i want to be ready... Like i said, alloys are not a problem in any position, but its like i forgot how to weld carbon when i try to bead the bottom...Last edited by Iveyrockstar; 02-01-2011 at 12:52 PM.
Reply:If need be, backfeed the wire on the bottom half first.  Since it wets out better than nickel/stainless alloys, you'll want just enough of a wiggle of the tungsten to break down the wire and root - a wide oscillation will result in suck back.  Applying pressure inwards with the filler while welding the hot pass can also help some.  I prefer to use 1/8 wire with a gap just tight enough to not let the wire fall through, but am just as comfortable opening it up just slightly to feed it.Just remember - flush is fine, and in some institutions, preferred.  We'll bust out a welder on any root reinforcement exceeding 3/32" typically.
Reply:Thanks for the tip... Ill try that.  And you're right.  I dont want too much bead. Its just that i want to try a method that ensures it's in there with no chance of suckback.
Reply:For starters use smaller diameter filler. I like 1/16" wire for most all my tig roots. Cram it in there and feed the wire on the sides not just the middle.If I were you I wouldn't go take a test and try to feed the filler a way that your not totally comfortable. Do what you know and try smaller filler.