You say barn finds. Have you gone through the planes and checked everything? This could be caused by several different things.

Try changing the glow plugs and make sure your glow driver is charged and working. I personally like the OS glow plugs, especailly for the four strokes. How are the fuel lines coming from the tank to the motor? Are they new or old? They could have holes in it allowing air into the line. This goes for the fuel line and vent line. The lines could look good, but it only takes one little hole to cause problems. Is the stopper in the fuel tank sealing properly? I would start there and see what happens. If you check those things and still does not run; you might need to clean the carbs and make sure you don't have trash in the needles.