[pitbull76]

I think the screw you found is one of three used for carb synchronization. You need a carb sync tool to get it right.
Also, are you sure that you have everything connected to the petcock properly? If you miss the vacuum hose, it won't run for sh#t. Try setting the petcock to prime temporarily and see if it runs right. If so, the vacuum isn't getting to the petcock.