RE: SureStart Questions
When you rotated the backplate, did you remove the choke tube to mount it? Two things can occur. The backplate screws may bottom out in the crankcase before the backplate tightens up -- the choketube adds enough thickness to keep this from happening. Secondly, when mounted flat without a choketube, some of the air intakes will be blocked due to molding flash and not allow airflow into the venturi. You can either Dremel a groove in the firewall to improve airflow or use an Xacto knife to cut away some of the flashing.
Like George said, check the back of the crankcase -- I've found many of the screw holes will have a shoulder from tapping. The case back can be smoothed up by light lapping with oil on a piece of 400 or 600 grit paped placed flat on a piece of glass.