Turn the GAV clockwise to close the Gas Adjustment Valve. If you have an original Zenith Carb then typically you open the valve a quarter to half for startup and then as the engine warms up then either fully closed to a quarter open. If you have a Tillitson aftermarket carb then you may have to open the Gav more for startup and may have to run it a quarter to half open depending on the condition of the carb. Every motor and carb are a bit different so you just have to play with it and find your carbs sweet spot. I find too many folks run the GAV more open than they should. If you pull the plugs and find them covered in black soot the you are running the carb too rich. Check out
www.modelabasics.com for some great details.