Air leakage into fuel lines is the most common cause of hard starting/hiccuping of the 606 engines-assuming the glow plug system is OK. More common in situations with low fuel in tank also. Has been covered in many old threads and can be researched using the "Search" mode. The rubber O-ring at the top of the prefilter is maybe the number one source of air intake because the new prefilters don't come with a new O-ring, so the old one gets recycled beyond its ability to function.