Yo Stay with the 7mm venturi. Its hard to tune over the web, but I will tell you my Orions are all set with the high speed and low speed set at about .5 mil in. Make sure the carb is not sticking, and make sure at neutral the carb is closing all the way. Set idle to about .75 mil. Make sure you don't have a hole in the carb boot and make sure the header is sealed on the gasket. Make sure there is good fuel line pressure. Because you cant get an idle makes me think there is an issue with fuel pressure.