Welding cast iron and carb question

[Eugene Harrison <eugh@2xtreme.net>]

I have a cast iron exhaust manifold from a '79 MGB. It has a small crack
(3/4") at the
crotch of the "U" turn where the catalitic converter bolts on. A friend
has a Heli Arc welder and will weld it for me. My question is what
welding rod to use and are there any other preporation or technique I
should be aware of?
      Also, the zenith carb keeps loosing its dampener oil. Ive had it
apart and i can't see what is to keep the oil in. No "O" ring etc. What
is the solution?
Thanks
 Eugene