I'll have to assume this is a car engine? There should be a brass or yellow thingy sticking up from the front of the engine. There should be a screw driver slot type screw in the end of the brass thing. This is your high speed needle. There is another Low speed needle on the side of the carb at the bottom of this brass thing. It has a screw driver slot too. But you should not have to touch the low speed screw. It might be locked down by a second screw on the side.

Enjoy,

Jim