Learning to read your glow plugs will help you learn to tune. Attached (hopefully they upload in the proper order) are several photos of glow plugs. They should appear as "too rich" (bottom of plug looks brand new), "just right" (nice gold color), "too lean" (bottom of plug is black, element may appear fuzzy or have a dull gray appearance), and "WAY too lean" (element crumpled, distorted, or broken element. Bottom of plug black with small pits). A gold color on the plug base and a shiny silver element is good. A slight grayish element and gold plug base means you're at peak mixture. Go no leaner than this setting.