I also am assuming that you are running solid wire downhill. And I will assume that your lack of fusion is between your passes near the bevel faces. You didn't mention what material thickness you are welding, which would affect the parameter selection, but by the sounds of it your parameters are too low or you are trying to carry to large a weld puddle. Qualification tests that I administer here at this company consist of GMAW-S (Short Arc) on pipe downhill. One of the tests I give is this process is on .814 thick, 6" diameter pipe, no preheat, fixed 5G, 100% CO2. GMAW all passes. I perform side bend tests on these specimens obviously. The seasoned welders have absolutely no problems with inter-pass Incomplete fusion. The new guys coming in (usually with a SMAW background) do have rashes of problems. Most seem to be the product of trying to carry too large a weld puddle downhill, which really should not exceed 1/8" thickness in a single deposit or they just don't add enough voltage to wet the toes out properly. The procedure allows for weaves or stringers - most weave to a width of about 5/8" per pass on fills and cap and overlap them. If you could give some more info it would be beneficial