With the exception of a vacuum leak, the idle is affected by the throttle valve being open. What you need to do is figure out what is making the valve stay open.

• With the engine warmed up is the choke valve open completely? If not, then it is keeping the throttle from closing.
• The 1st thing to do is remove the carburetor from the engine.
• Now with the linkage eliminated pump the throttle. Does the throttle valve close completely?
• Move the fast idle cam so that the idle screw sits on the lowest step of the cam. The throttle valve should be closed. If not, then the fast idle cam might be installed wrong, or is sticking. This will keep the carburetor from getting to slow idle.
• Turn the idle screw out until it isn't touching the fast idle cam. Check the throttle valve now. Keep in mind that the idle screw opens the throttle valve, which speeds up the RPM.
• Disconnect the link that connects the throttle shaft to the accelerator diaphragm arm. Work the throttle to see if the throttle plate closes. If it does then you probably have something wrong with the diaphragm setup.
• If it all looks good, then take the top off and check the Accelerator pump diaphragm to see if it is put together correctly. See this page to learn about the accelerator pump diaphragm.