Here is an example of what I am talking about. You hook it to you carburetor when you are adjusting the idle mixture. They have them at Advance or Autozone. Most people don't realize that at low RPM cruising, your idle mixture is a large part of your running mixture. Too lean and you will get surging and too rich you get sputtering, smoke, etc.
http://www.summitracing.com/parts/WMR-W80594/?rtype=10
http://www.summitracing.com/parts/WMR-W80594/?rtype=10